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/Common/Common.h

@@ -136,6 +136,7 @@ enum HOST_COMM
 	WM_USER_STOP = 10,
 	WM_USER_CREATE,
 	WM_USER_SETCURSOR,
+	WM_USER_QUIT,
 };
 
 // Used for notification on emulation state