SDL: handle SDL_QUIT event

Using SDL_INIT_JOYSTICK implies SDL_INIT_EVENTS which installs a signal
handler for SIGINT and SIGTERM. There will be a way to prevent this in
2.0.4 but for now we'll need to handle SDL_QUIT.

Source/Core/DolphinWX/MainNoGUI.cpp

@@ -49,7 +49,7 @@ void Host_RefreshDSPDebuggerWindow() {}
 static Common::Event updateMainFrameEvent;
 void Host_Message(int Id)
 {
-	if (Id == WM_USER_STOP)
+	if (Id == WM_USER_STOP || Id == WM_USER_QUIT)
 		running = false;
 }