contrib/bc: fix build with GCC

Building with GCC failed with the following error message:

error: to be safe all intermediate pointers in cast from 'char '
to 'const char ' must be 'const' qualified [-Werror=cast-qual]

This was caused by main() being declared with "char *argv[]" as the
3rd parameter, but argv later being passed cast to "const char**":

113 | if (BC_IS_BC) s = bc_main(argc, (const char**) argv);

|                                      ^

This is fixed by declaring the 3rd parameter of main() as "const
char *argv[]".

Reported by: CI
MFC after: 3 days