Core: Use override consistently

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

Change-Id: If33399d60e96cae766bbedbe30044ada411e862f
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/messagemanager.h

@@ -61,7 +61,7 @@ public slots:
 
 private:
     MessageManager();
-    ~MessageManager();
+    ~MessageManager() override;
     static void init();
     friend class Core::Internal::MainWindow;
 };