pthread_mutex_t is a CRITICAL_SECTION (not a pointer) on Win32

Partially revert Deuce's commit b4bc9c97

This explains the EAccessViolation (stack corruption during sbbs startup) that sysops have reported when using the latest nightly builds.

src/xpdev/threadwrap.h

@@ -75,7 +75,7 @@ extern "C" {
 
 	#else	/* Implemented as Win32 Critical Sections */
 
-		typedef intptr_t pthread_mutex_t;
+		typedef CRITICAL_SECTION pthread_mutex_t;
 
 	#endif