games/stratagus: fix build on powerpc

/usr/bin/c++ -DHAVE_GETOPT -DHAVE_STRCASESTR -DHAVE_STRNLEN -DPIXMAPS=\"/usr/local/share/pixmaps\" -DUSE_BSD -DUSE_BZ2LIB -DUSE_MNG -DUSE_THEORA -DUSE_VORBIS -DUSE_ZLIB -I/wrkdirs/usr/ports/games/stratagus/work/stratagus-3.2.0/src/include -I/wrkdirs/usr/ports/games/stratagus/work/stratagus-3.2.0/src/guichan/include -I/wrkdirs/usr/ports/games/stratagus/work/stratagus-3.2.0/src/guichan/include/guichan -I/wrkdirs/usr/ports/games/stratagus/work/stratagus-3.2.0/src/third_party -I/wrkdirs/usr/ports/games/stratagus/work/.build -I/usr/local/include/lua51 -I/usr/local/include/SDL2 -I/usr/local/include -O2 -pipe -fstack-protector-strong -fno-strict-aliasing  -Wno-register -fsigned-char -O2 -pipe -fstack-protector-strong -fno-strict-aliasing  -Wno-register -std=gnu++17 -MD -MT CMakeFiles/stratagus.dir/src/map/fow.cpp.o -MF CMakeFiles/stratagus.dir/src/map/fow.cpp.o.d -o CMakeFiles/stratagus.dir/src/map/fow.cpp.o -c
/wrkdirs/usr/ports/games/stratagus/work/stratagus-3.2.0/src/map/fow.cpp
/wrkdirs/usr/ports/games/stratagus/work/stratagus-3.2.0/src/map/fow.cpp:39:10: fatal error: 'omp.h' file not found
         ^~~~~~~