libgfortran: Fix build for targets with int32_t=long int

libgfortran: Fix build for targets with int32_t=long int

Without this, after r15-6415-g586477d67bf2e3, you'll see,
for targets where int32_t is a typedef of long int (beware
of artificially broken lines):

/x/gcc/libgfortran/caf/single.c: In function '_gfortran_caf_get_by_ct':
/x/gcc/libgfortran/caf/single.c:2943:56: error: passing argument 2 of '\
(accessor_hash_table + (sizetype)((unsigned int)getter_index * 12))->ac\
cessor' from incompatible pointer type [-Wincompatible-pointer-types]
 2943 |   accessor_hash_table[getter_index].accessor (dst_ptr, &free_bu\
ffer, src_ptr,
      |                                                        ^~~~~~~~\
~~~~
      |                                                        |
      |                                                        int *
/x/gcc/libgfortran/caf/single.c:2943:56: note: expected 'int32_t *' {ak\
a 'long int *'} but argument is of type 'int *'

libgfortran:
	* caf/single.c (_gfortran_caf_get_by_ct): Correct type of free_buffer
	to int32_t.