Replace WIN32 with _WIN32

Using WIN32 depends on the build environment defining the variable.
_WIN32 is a predefined MSVC macro and is always available.

Signed-off-by: Tim Crawford <crawfxrd@gmail.com>

lib/sync.c

@@ -27,7 +27,7 @@
 #include "aros/aros_compat.h"
 #endif
 
-#if defined(WIN32)
+#if defined(_WIN32)
 #include <winsock2.h>
 #include "win32/win32_compat.h"
 #endif