Fixed windows compilation failure introduced by fix for BUG#12982.

include/my_global.h:
  Remove cast to ssize_t, since there is no ssize_t type on Windows.

include/my_global.h

@@ -737,7 +737,7 @@ typedef SOCKET_SIZE_TYPE size_socket;
 #define FLT_MAX		((float)3.40282346638528860e+38)
 #endif
 #ifndef SSIZE_MAX
-#define SSIZE_MAX (ssize_t)((~((size_t) 0)) / 2)
+#define SSIZE_MAX ((~((size_t) 0)) / 2)
 #endif
 
 #if !defined(HAVE_ISINF) && !defined(isinf)