Factor out UNUSED macro to header file

This is so it can be used in other files without having to copy-paste
the define.

desktop_version/src/Network.c

@@ -1,8 +1,7 @@
 #include <stdint.h>
 
 #include "MakeAndPlay.h"
-
-#define UNUSED(expr) (void)(expr)
+#include "Unused.h"
 
 #ifdef MAKEANDPLAY
 	#ifdef STEAM_NETWORK

desktop_version/src/Unused.h

@@ -0,0 +1,6 @@
+#ifndef UNUSED_H
+#define UNUSED_H
+
+#define UNUSED(expr) (void)(expr)
+
+#endif /* UNUSED_H */