build: Fix 'incompatible pointer types' warning on i686

This fixes a recent Fedora build, which failed on i686 architecture
with "incompatible pointer types" error:

libpst.c: In function 'pst_read_block_size':
libpst.c:3832:36: error: passing argument 2 of 'uncompress' from incompatible pointer type [-Wincompatible-pointer-types]
3832 | if (uncompress((Bytef *) *buf, &result_size, (Bytef *) zbuf, size) != Z_OK || result_size != inflated_size) {

|                                    ^~~~~~~~~~~~
|                                    |
|                                    size_t * {aka unsigned int *}

In file included from libpst.c:9:
/usr/include/zlib.h:1251:70: note: expected 'long unsigned int *' but argument is of type 'size_t *' {aka 'unsigned int *'}
1251 | Z_EXTERN int Z_EXPORT uncompress(unsigned char *dest, unsigned long *destLen, const unsigned char *source, unsigned long sourceLen);

|                                                       ~~~~~~~~~~~~~~~^~~~~~~

Fixes: commit a9fb0d8c21c781e679e6e93bb24da14b620ce60d