Fix clang warning.

See merge request !1340

GaudiKernel/include/GaudiKernel/DataObjID.h

 /***********************************************************************************\
-* (c) Copyright 1998-2020 CERN for the benefit of the LHCb and ATLAS collaborations *
+* (c) Copyright 1998-2022 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".                                            *
@@ -54,6 +54,8 @@ public:
   DataObjID( const CLID& clid, std::string key );
   DataObjID( std::string className, std::string key );
 
+  DataObjID& operator=( const DataObjID& ) = default;
+
   /// only return the last part of the key
   const std::string& key() const { return m_key; }