Fix: Treat closing network relay window as a choice of "No".

Issue ConnectFailure when closing the window if the relay request is
considered unhandled.

src/network/network_gui.h

@@ -39,5 +39,9 @@ struct NetworkCompanyInfo : NetworkCompanyStats {
 	std::string clients;      ///< The clients that control this company (Name1, name2, ..)
 };
 
+enum NetworkRelayWindowCloseData {
+	NRWCD_UNHANDLED = 0, ///< Relay request is unhandled.
+	NRWCD_HANDLED = 1, ///< Relay request is handled, either by user or by timeout.
+};
 
 #endif /* NETWORK_GUI_H */