msvc: Move `ssize_t` typedef to MSVC-only

This is a MSVC-only issue. All other compilers we support work properly.

src/common.h

@@ -33,10 +33,6 @@
 
 # define snprintf _snprintf
 
-#ifndef _SSIZE_T_DEFINED
-typedef SSIZE_T ssize_t;
-#endif
-
 #else
 # include <unistd.h>
 

src/win32/msvc-compat.h

@@ -33,6 +33,9 @@
 #	define strcasecmp	_stricmp
 #	define strncasecmp _strnicmp
 
+/* MSVC doesn't define ssize_t at all */
+typedef SSIZE_T ssize_t;
+
 #endif
 
 #endif /* INCLUDE_msvc_compat__ */