Add private tool to SvcWithTool example

GaudiExamples/src/AlgTools/MyServiceWithTool.cpp

 /***********************************************************************************\
-* (c) Copyright 1998-2019 CERN for the benefit of the LHCb and ATLAS collaborations *
+* (c) Copyright 1998-2021 CERN for the benefit of the LHCb and ATLAS collaborations *
 *                                                                                   *
 * This software is distributed under the terms of the Apache version 2 licence,     *
 * copied verbatim in the file "LICENSE".                                            *
@@ -22,6 +22,7 @@ namespace GaudiTesting {
 
   private:
     PublicToolHandle<IMyTool> m_tool{this, "MyTool", "MyTool"};
+    ToolHandle<IMyTool> m_privTool{this, "MyPrivateTool", ""};
   };
 
   DECLARE_COMPONENT( SvcWithTool )