Fix FTBFS with GCC 15.

[sebastic]

As reported in Debian Bug #1097191, libgeotiff fails to build with GCC 15:

geotifcp.c: In function ‘main’:
geotifcp.c:114:21: error: too many arguments to function ‘getopt’; expected 0, have 3
  114 |         while ((c = getopt(argc, argv, "c:f:l:o:p:r:w:e:g:4:v:aistd8BLMC")) != -1)
      |                     ^~~~~~ ~~~~
geotifcp.c:40:5: note: declared here
   40 | int getopt();
      |     ^~~~~~

See also: https://gcc.gnu.org/gcc-15/porting_to.html#c23-fn-decls-without-parameters