IMPORT: slz: silence a build warning on non-x86 non-arm

IMPORT: slz: silence a build warning on non-x86 non-arm

Building with clang 16 on MIPS64 yields this warning:

  src/slz.c:931:24: warning: unused function 'crc32_uint32' [-Wunused-function]
  static inline uint32_t crc32_uint32(uint32_t data)
                         ^

Let's guard it using UNALIGNED_LE_OK which is the only case where it's
used. This saves us from introducing a possibly non-portable attribute.

This is libslz upstream commit f5727531dba8906842cb91a75c1ffa85685a6421.