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/qt/optionsmodel.cpp

@@ -106,11 +106,7 @@ void OptionsModel::Init()
 
     // Network
     if (!settings.contains("fUseUPnP"))
-#ifdef USE_UPNP
-        settings.setValue("fUseUPnP", true);
-#else
-        settings.setValue("fUseUPnP", false);
-#endif
+        settings.setValue("fUseUPnP", DEFAULT_UPNP);
     if (!SoftSetBoolArg("-upnp", settings.value("fUseUPnP").toBool()))
         addOverriddenOption("-upnp");