misc/mnn: Fix build

FAILED: tools/converter/CMakeFiles/MNNConvertDeps.dir/__/__/3rd_party/flatbuffers/src/util.cpp.o
/usr/local/libexec/ccache/c++ -DGOOGLE_PROTOBUF_NO_RTTI -DMNNConvertDeps_EXPORTS -DMNN_SUPPORT_TFLITE_QUAN -DMNN_USE_THREAD_POOL -DMNN_VERSION=\"0.2.1.5git\" -DMNN_VERSION_MAJOR=0 -DMNN_VERSION_MINOR=2 -DMNN_VERSION_PATCH=1 -I/wrkdirs/usr/ports/misc/mnn/work/MNN-1.2.0/include -I/wrkdirs/usr/ports/misc/mnn/work/MNN-1.2.0/source -I/wrkdirs/usr/ports/misc/mnn/work/MNN-1.2.0/express -I/wrkdirs/usr/ports/misc/mnn/work/MNN-1.2.0/tools -I/wrkdirs/usr/ports/misc/mnn/work/MNN-1.2.0/codegen -I/wrkdirs/usr/ports/misc/mnn/work/MNN-1.2.0/schema/current -I/wrkdirs/usr/ports/misc/mnn/work/MNN-1.2.0/3rd_party -I/wrkdirs/usr/ports/misc/mnn/work/MNN-1.2.0/3rd_party/half -I/wrkdirs/usr/ports/misc/mnn/work/MNN-1.2.0/3rd_party/imageHelper -I/wrkdirs/usr/ports/misc/mnn/work/MNN-1.2.0/3rd_party/OpenCLHeaders -I/wrkdirs/usr/ports/misc/mnn/work/MNN-1.2.0/tools/converter/include -I/wrkdirs/usr/ports/misc/mnn/work/MNN-1.2.0/tools/converter/source/tflite/schema -I/wrkdirs/usr/ports/misc/mnn/work/.build/tools/converter -O2 -pipe -fPIC -fstack-protector-strong -isystem /usr/local/include -fno-strict-aliasing  -pthread -fPIC -isystem /usr/local/include -std=c++11 -O3 -fvisibility-inlines-hidden -fvisibility=hidden -fomit-frame-pointer -fstrict-aliasing -ffunction-sections -fdata-sections -ffast-math -fno-rtti -fno-exceptions  -O2 -pipe -fPIC -fstack-protector-strong -isystem /usr/local/include -fno-strict-aliasing  -pthread -fPIC -isystem /usr/local/include -fPIC -std=gnu++11 -MD -MT tools/converter/CMakeFiles/MNNConvertDeps.dir/__/__/3rd_party/flatbuffers/src/util.cpp.o -MF tools/converter/CMakeFiles/MNNConvertDeps.dir/__/__/3rd_party/flatbuffers/src/util.cpp.o.d -o tools/converter/CMakeFiles/MNNConvertDeps.dir/__/__/3rd_party/flatbuffers/src/util.cpp.o -c /wrkdirs/usr/ports/misc/mnn/work/MNN-1.2.0/3rd_party/flatbuffers/src/util.cpp
/wrkdirs/usr/ports/misc/mnn/work/MNN-1.2.0/3rd_party/flatbuffers/src/util.cpp:150:37: error: use of undeclared identifier 'string_back'
    char &filepath_last_character = string_back(filepath);
                                    ^
/wrkdirs/usr/ports/misc/mnn/work/MNN-1.2.0/3rd_party/flatbuffers/src/util.cpp:227:18: error: use of undeclared identifier 'string_back'
          (ch == string_back(s)))
                 ^
2 errors generated.

Approved by:	portmgr (blanket)

misc/mnn/files/patch-3rd__party_flatbuffers_src_util.cpp

+Reference:	https://github.com/google/flatbuffers/commit/6748c373be9ef54e221265a502a3cee3bbf8cef1
+
+FAILED: tools/converter/CMakeFiles/MNNConvertDeps.dir/__/__/3rd_party/flatbuffers/src/util.cpp.o 
+/usr/local/libexec/ccache/c++ -DGOOGLE_PROTOBUF_NO_RTTI -DMNNConvertDeps_EXPORTS -DMNN_SUPPORT_TFLITE_QUAN -DMNN_USE_THREAD_POOL -DMNN_VERSION=\"0.2.1.5git\" -DMNN_VERSION_MAJOR=0 -DMNN_VERSION_MINOR=2 -DMNN_VERSION_PATCH=1 -I/wrkdirs/usr/ports/misc/mnn/work/MNN-1.2.0/include -I/wrkdirs/usr/ports/misc/mnn/work/MNN-1.2.0/source -I/wrkdirs/usr/ports/misc/mnn/work/MNN-1.2.0/express -I/wrkdirs/usr/ports/misc/mnn/work/MNN-1.2.0/tools -I/wrkdirs/usr/ports/misc/mnn/work/MNN-1.2.0/codegen -I/wrkdirs/usr/ports/misc/mnn/work/MNN-1.2.0/schema/current -I/wrkdirs/usr/ports/misc/mnn/work/MNN-1.2.0/3rd_party -I/wrkdirs/usr/ports/misc/mnn/work/MNN-1.2.0/3rd_party/half -I/wrkdirs/usr/ports/misc/mnn/work/MNN-1.2.0/3rd_party/imageHelper -I/wrkdirs/usr/ports/misc/mnn/work/MNN-1.2.0/3rd_party/OpenCLHeaders -I/wrkdirs/usr/ports/misc/mnn/work/MNN-1.2.0/tools/converter/include -I/wrkdirs/usr/ports/misc/mnn/work/MNN-1.2.0/tools/converter/source/tflite/schema -I/wrkdirs/usr/ports/misc/mnn/work/.build/tools/converter -O2 -pipe -fPIC -fstack-protector-strong -isystem /usr/local/include -fno-strict-aliasing  -pthread -fPIC -isystem /usr/local/include -std=c++11 -O3 -fvisibility-inlines-hidden -fvisibility=hidden -fomit-frame-pointer -fstrict-aliasing -ffunction-sections -fdata-sections -ffast-math -fno-rtti -fno-exceptions  -O2 -pipe -fPIC -fstack-protector-strong -isystem /usr/local/include -fno-strict-aliasing  -pthread -fPIC -isystem /usr/local/include -fPIC -std=gnu++11 -MD -MT tools/converter/CMakeFiles/MNNConvertDeps.dir/__/__/3rd_party/flatbuffers/src/util.cpp.o -MF tools/converter/CMakeFiles/MNNConvertDeps.dir/__/__/3rd_party/flatbuffers/src/util.cpp.o.d -o tools/converter/CMakeFiles/MNNConvertDeps.dir/__/__/3rd_party/flatbuffers/src/util.cpp.o -c /wrkdirs/usr/ports/misc/mnn/work/MNN-1.2.0/3rd_party/flatbuffers/src/util.cpp
+/wrkdirs/usr/ports/misc/mnn/work/MNN-1.2.0/3rd_party/flatbuffers/src/util.cpp:150:37: error: use of undeclared identifier 'string_back'
+    char &filepath_last_character = string_back(filepath);
+                                    ^
+/wrkdirs/usr/ports/misc/mnn/work/MNN-1.2.0/3rd_party/flatbuffers/src/util.cpp:227:18: error: use of undeclared identifier 'string_back'
+          (ch == string_back(s)))
+                 ^
+2 errors generated.
+
+--- 3rd_party/flatbuffers/src/util.cpp.orig	2021-06-11 09:17:13 UTC
++++ 3rd_party/flatbuffers/src/util.cpp
+@@ -147,7 +147,7 @@ std::string ConCatPathFileName(const std::string &path
+                                const std::string &filename) {
+   std::string filepath = path;
+   if (filepath.length()) {
+-    char &filepath_last_character = string_back(filepath);
++    char &filepath_last_character = filepath.back();
+     if (filepath_last_character == kPathSeparatorWindows) {
+       filepath_last_character = kPathSeparator;
+     } else if (filepath_last_character != kPathSeparator) {
+@@ -224,7 +224,7 @@ ClassicLocale ClassicLocale::instance_;
+ std::string RemoveStringQuotes(const std::string &s) {
+   auto ch = *s.c_str();
+   return ((s.size() >= 2) && (ch == '\"' || ch == '\'') &&
+-          (ch == string_back(s)))
++          (ch == s.back()))
+              ? s.substr(1, s.length() - 2)
+              : s;
+ }