graphics/poppler: fix build with libc++ 19

As noted in the libc++ 19 release notes [1], std::char_traits<> is now
only provided for char, char8_t, char16_t, char32_t and wchar_t, and any
instantiation for other types will fail.

This causes graphics/poppler to fail to compile with clang 19 and libc++
19, resulting in errors similar to:

    /usr/include/c++/v1/string:820:42: error: implicit instantiation of undefined template 'std::char_traits<unsigned short>'
      820 |   static_assert(is_same<_CharT, typename traits_type::char_type>::value,
          |                                          ^
    /wrkdirs/usr/ports/graphics/poppler/work/poppler-24.02.0/cpp/poppler-global.h:101:43: note: in instantiation of template class 'std::basic_string<unsigned short>' requested here
      101 | class POPPLER_CPP_EXPORT ustring : public std::basic_string<unsigned short>
          |                                           ^
    /usr/include/c++/v1/__fwd/string.h:23:29: note: template is declared here
       23 | struct _LIBCPP_TEMPLATE_VIS char_traits;
          |                             ^

Upstream poppler has fixed this in commit b4ac7d9a [2], so add it as a
PATCHFILES entry in the Makefile, until the next version of poppler is
released.

[1] https://libcxx.llvm.org/ReleaseNotes/19.html#deprecations-and-removals
[2] https://gitlab.freedesktop.org/poppler/poppler/-/commit/b4ac7d9a

PR:		280564
Approved by:	maintainer timeout (2 weeks)
MFH:		2024Q3

(cherry picked from commit 6bf6ee19)