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/aggregation/aggregate.h

@@ -41,7 +41,7 @@ class AGGREGATION_EXPORT Aggregate : public QObject
 
 public:
     Aggregate(QObject *parent = 0);
-    virtual ~Aggregate();
+    ~Aggregate() override;
 
     void add(QObject *component);
     void remove(QObject *component);