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/cpptools/builtincursorinfo.cpp

@@ -123,7 +123,7 @@ public:
     }
 
 protected:
-    virtual bool preVisit(AST *ast)
+    bool preVisit(AST *ast) override
     {
         if (m_functionDefinition)
             return false;