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/InputCommon/ControllerInterface/SDL/SDL.h

@@ -25,6 +25,7 @@ namespace SDL
 {
 
 void Init( std::vector<Core::Device*>& devices );
+void UpdateInput();
 
 class Joystick : public Core::Device
 {
@@ -130,8 +131,6 @@ private:
 #endif
 
 public:
-	void UpdateInput() override;
-
 	Joystick(SDL_Joystick* const joystick, const int sdl_index, const unsigned int index);
 	~Joystick();