clang-tidy: Apply modernize-use-override everywhere (#6439)

... to mark overriding functions `override` instead of `virtual`.

plugins/Flanger/FlangerControlsDialog.h

@@ -42,7 +42,7 @@ class FlangerControlsDialog : public EffectControlDialog
 	Q_OBJECT
 public:
 	FlangerControlsDialog( FlangerControls* controls );
-	virtual ~FlangerControlsDialog()
+	~FlangerControlsDialog() override
 	{
 	}
 };