Common: extract UPnP namespace from NetPlayServer

Source/Core/Common/UPnP.h

@@ -0,0 +1,17 @@
+// Copyright 2017 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#pragma once
+
+#ifdef USE_UPNP
+
+#include "Common/CommonTypes.h"
+
+namespace UPnP
+{
+void TryPortmapping(u16 port);
+void StopPortmapping();
+}
+
+#endif