Prefer using 'override' instead of 'virtual'

warning: prefer using 'override' or (rarely) 'final' instead of
'virtual' [modernize-use-override]

Change-Id: I6dac7a62b627fa1353b4455e1af92f869c2571cc
Reviewed-by: Marco Benelli <marco.benelli@qt.io>

src/plugins/designer/codemodelhelpers.cpp

@@ -64,7 +64,7 @@ public:
     explicit SearchFunction(const char *name);
     FunctionList operator()(const DocumentPtr &doc);
 
-    virtual bool visit(CPlusPlus::Function * f);
+    bool visit(CPlusPlus::Function * f) override;
 
 private:
     const size_t m_length;