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/stringtable.cpp

@@ -58,7 +58,7 @@ public:
 
     public:
         GCRunner(StringTablePrivate &stringTable): m_stringTable(stringTable) {}
-        virtual void run() { m_stringTable.GC(); }
+        void run() override { m_stringTable.GC(); }
     } m_gcRunner;
 
     mutable QMutex m_lock;