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>

include/md5.h

@@ -23,7 +23,7 @@
 #ifndef MD5_H
 #define MD5_H
 
-#if defined(WIN32)
+#if defined(_WIN32)
 #else
 #include <netinet/in.h>
 #endif