ExtensionSystem/Aggregation: Use override consistently

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

Change-Id: Ic10099c2a2f3a070c972442736e50e797a063597
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/libs/extensionsystem/iplugin.h

@@ -51,7 +51,7 @@ public:
     };
 
     IPlugin();
-    virtual ~IPlugin();
+    ~IPlugin() override;
 
     virtual bool initialize(const QStringList &arguments, QString *errorString) = 0;
     virtual void extensionsInitialized() = 0;