Fix printf compiler warning

[rfl890]

An incorrect format specifier is used in cache.c (%lu instead of %zu for size_t) causing a compiler warning in Clang:

C:/Users/RFL890/zopfli/src/zopfli/cache.c:37:9: warning: format specifies type 'unsigned long' but the argument has type 'size_t' (aka 'unsigned long long') [-Wformat]
   36 |         "Error: Out of memory. Tried allocating %lu bytes of memory.\n",
      |                                                 ~~~
      |                                                 %zu
   37 |         (unsigned long)ZOPFLI_CACHE_LENGTH * 3 * blocksize);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 warning generated.

This PR fixes it by using the correct format specifier