walloc: fix size type from int => size_t

[lumpidu]

The type size_t is used for most moderm implementations of malloc, calloc, realloc, etc.

Using just signed types like int leads to issues on 32-bit systems, if there are really large chunks of memory used such as e.g. for a chunk of 3.2 GB in Festival:

Building clunits/ldom index
WALLOC: failed to malloc -1094967296 bytes

Also, the macros walloc(), wcalloc() & wrealloc() use sizeof(), which itself returns size_t and not int.

Compiling speech-tools and Festival with these changes, at least compiles on my Mac just fine.