Fix type mismatch between declaration and definition

[aleasto]

Compiling with LTO otherwise fails with:

wg_mempool.h:91:18: error: type of 'wgmempool_Init' does not match original declaration [-Werror=lto-type-mismatch]
   91 |     extern void *wgmempool_Init(uint4 blocksize, size_t maxstrsize);
      |                  ^
wg_mempool.c:86:14: note: type mismatch in parameter 1
   86 | extern void *wgmempool_Init(size_t blocksize, size_t maxstrsize)
      |              ^
wg_mempool.c:86:14: note: type 'size_t' should match type 'uint4'
wg_mempool.c:86:14: note: 'wgmempool_Init' was previously declared here
wg_mempool.c:86:14: note: code may be misoptimized unless '-fno-strict-aliasing' is used