Add #ifdef USE_UPNP around usage of fUseUPnP to fix build failure.

src/main.cpp

@@ -65,13 +65,11 @@ int fLimitProcessors = false;
 int nLimitProcessors = 1;
 int fMinimizeToTray = true;
 int fMinimizeOnClose = true;
-#ifdef USE_UPNP
 #if USE_UPNP
 int fUseUPnP = true;
 #else
 int fUseUPnP = false;
 #endif
-#endif