Qt: Fix volume slider slowing down emulation

src/duckstation-qt/audiosettingswidget.h

@@ -18,6 +18,8 @@ public:
 private Q_SLOTS:
   void updateBufferingLabel();
   void updateVolumeLabel();
+  void onOutputVolumeChanged(int new_value);
+  void onOutputMutedChanged(int new_state);
 
 private:
   Ui::AudioSettingsWidget m_ui;