CaloClusterMatching: Fix clang warnings.

Clang warnings:  missing override keywords.

Reconstruction/RecoTools/CaloClusterMatching/src/CaloClusterMatchingTool.h

 ///////////////////////// -*- C++ -*- /////////////////////////////
 
 /*
-  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2018 CERN for the benefit of the ATLAS collaboration
 */
 
 // CaloClusterMatchingTool.h 
@@ -48,8 +48,8 @@ namespace ClusterMatching {
     virtual ~CaloClusterMatchingTool(); 
 
     // Athena algtool's Hooks
-    virtual StatusCode  initialize();
-    virtual StatusCode  finalize();
+    virtual StatusCode  initialize() override;
+    virtual StatusCode  finalize() override;
 
     /////////////////////////////////////////////////////////////////// 
     // Const methods: