Silence some cppcheck warnings

ObjectTypes/Skeleton/Properties.hh

@@ -141,9 +141,9 @@ protected:
       * Called by the Properties container to keep the indices in sync.
       */
     //@{
-    void insert_at(int _index);
-    void remove_at(int _index);
-    void clear();
+    void insert_at(int _index) override;
+    void remove_at(int _index) override;
+    void clear() override;
     //@}
 
   protected: