graphics/khronos-texture: Fix build with libc++ 21

[...]
In file included from /wrkdirs/usr/ports/graphics/khronos-texture/work/KTX-Software-4.4.2/external/fmt/src/os.cc:13:
In file included from /wrkdirs/usr/ports/graphics/khronos-texture/work/KTX-Software-4.4.2/external/fmt/include/fmt/os.h:11:
/wrkdirs/usr/ports/graphics/khronos-texture/work/KTX-Software-4.4.2/external/fmt/include/fmt/format.h:747:28: error: use of undeclared identifier 'malloc'

747 |     T* p = static_cast<T*>(malloc(n * sizeof(T)));
    |                            ^~~~~~

/wrkdirs/usr/ports/graphics/khronos-texture/work/KTX-Software-4.4.2/external/fmt/include/fmt/format.h:752:35: error: use of undeclared identifier 'free'

752 |   void deallocate(T* p, size_t) { free(p); }
    |                                   ^~~~

[...]

While here udpate to USES+=compiler:c++14-lang - port uses -std=c++14.

PR: 293197
MFH: 2026Q1
(cherry picked from commit 438a70797fc27940aa3276972591444707792551)