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/mainwindow.h

@@ -76,7 +76,7 @@ class MainWindow : public Utils::AppMainWindow
 
 public:
     MainWindow();
-    ~MainWindow();
+    ~MainWindow() override;
 
     void init();
     void extensionsInitialized();
@@ -112,7 +112,7 @@ public slots:
     void exit();
 
 protected:
-    virtual void closeEvent(QCloseEvent *event);
+    void closeEvent(QCloseEvent *event) override;
 
 private:
     void openFile();