add DEFAULT_UPNP constant in net

- as this is a shared Core/GUI setting, this makes it easier to keep them
  in sync (also no new includes are needed)

src/net.cpp

@@ -1736,10 +1736,8 @@ void StartNode(boost::thread_group& threadGroup)
     else
         threadGroup.create_thread(boost::bind(&TraceThread<void (*)()>, "dnsseed", &ThreadDNSAddressSeed));
 
-#ifdef USE_UPNP
     // Map ports with UPnP
-    MapPort(GetBoolArg("-upnp", USE_UPNP));
-#endif
+    MapPort(GetBoolArg("-upnp", DEFAULT_UPNP));
 
     // Send and receive from sockets, accept connections
     threadGroup.create_thread(boost::bind(&TraceThread<void (*)()>, "net", &ThreadSocketHandler));