Help: Use override consistently

clang-tidy fixes from modernize-use-override check.

Change-Id: I3aec6d7ab1e8f088b745d2d9730e3d49ced42031
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/help/helpviewer.h

@@ -49,7 +49,7 @@ public:
     Q_DECLARE_FLAGS(Actions, Action)
 
     explicit HelpViewer(QWidget *parent = nullptr);
-    ~HelpViewer();
+    ~HelpViewer() override;
 
     virtual QFont viewerFont() const = 0;
     virtual void setViewerFont(const QFont &font) = 0;