fix improper integer type on systems with non-32-bit int

[mrpapersonic]

generally the parameter here should be utf8proc_int32_t anyway, but changing public facing APIs for something like this isn't a very good idea. maybe this should be changed for the next major bump in the libtool version

[stevengj]

seqindex_write_char_decomposed is not part of the API, so we can just change the parameter.