Fixes warnings regarding unhandled events.

The adding of the Windows tray notification enabled this event on
Windows. Since the event is not needed, nor handled on Windows for
GUI2 it caused a warning.

Untested, since I've no access to Windows, anonymissimus please test.

src/gui/auxiliary/event/handler.cpp

@@ -379,6 +379,10 @@ void thandler::handle_event(const SDL_Event& event)
 			handle_system_event(event);
 			break;
 		}
+#elif defined _WIN32
+		case SDL_SYSWMEVENT:
+			/* DO NOTHING */
+			break;
 #endif
 
 		case SDL_ACTIVEEVENT: