compat: define ssize_t as int on 32bit Windows, silences C4142 warning

It appears Python.h provides ssize_t, which is aliased to int.

https://hg.python.org/cpython/file/v2.7.11/PC/pyconfig.h#l205

mercurial/compat.h

@@ -8,7 +8,7 b''
 #if defined(_WIN64)
 typedef __int64 ssize_t;
 #else
-typedef long ssize_t;
+typedef int ssize_t;
 #endif
 typedef signed char int8_t;
 typedef short int16_t;