Make fails with GCC-15/16

[Pritchy96]

Hi all,

I seem to be having an issue compiling with GCC-15/16. Both give the same issue - I initially tried 16 as that's my system default, but then after seeing a commit for gcc-15 support I gave that a shot. Both give the issue:

[tom@tom-xps15 ps2sdk]$ make CC=gcc-15

...

src/romimg.c: In function ‘AddFile’:
src/romimg.c:471:27: error: initialization discards ‘const’ qualifier from pointer target type [-Werror=discarded-qualifiers]
  471 |                 char* T = strrchr(fname, '.');
      |                           ^~~~~~~
cc1: all warnings being treated as errors

I've managed to get it moving by running make cc=GCC-11, but, again, it appears from the above PR that up to at least GCC-15 is supported. I'm very possibly missing something, mea culpa if so, I'm a touch rusty in terms of playing with GCC.