UI: Fix mixer dock widget minSize being too big

When the vertical volume meter was introduced it set in the OBSBasic.ui
mixer dock definition a min width/height that was valid for both
widgets. QStackedWidget hints the minimum size as the higher w/h
of its children so this workaround is necessary.

UI/window-basic-main.hpp

@@ -252,6 +252,7 @@ private:
 	void GetAudioSourceProperties();
 	void VolControlContextMenu();
 	void ToggleVolControlLayout();
+	void ToggleMixerLayout(bool vertical);
 
 	void RefreshSceneCollections();
 	void ChangeSceneCollection();