math/gmp: fix build after base cd0727ec709b

After base cd0727ec709bb54f8f82104f6113284a15dd3464 ("libc: Add
<stdio.h> C23 feature test macro"), math/gmp fails to build with errors
similar to:

  inp_str.c:63:10: warning: call to undeclared function
'__gmpz_inp_str_nowhite'; ISO C99 and later do not support implicit function
declarations [-Wimplicit-function-declaration]
     63 |   return mpz_inp_str_nowhite (x, stream, base, c, nread);
        |          ^
  ../gmp-impl.h:1782:29: note: expanded from macro 'mpz_inp_str_nowhite'
   1782 | #define mpz_inp_str_nowhite __gmpz_inp_str_nowhite
        |                             ^
  inp_str.c:68:1: error: conflicting types for '__gmpz_inp_str_nowhite'
     68 | mpz_inp_str_nowhite (mpz_ptr x, FILE *stream, int base, int c, size_t
nread)
        | ^