InDetGlobalMonitoring: Fix clang warnings.

    
Don't use references in a range-for if the object must be copised.

InnerDetector/InDetMonitoring/InDetGlobalMonitoring/src/InDetGlobalTrackMonTool.cxx

 /*
-  Copyright (C) 2002-2020 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2021 CERN for the benefit of the ATLAS collaboration
 */
 
 /** @file InDetGlobalTrackMonTool.cxx
@@ -946,7 +946,7 @@ void InDetGlobalTrackMonTool::FillTIDE()
 		    const xAOD::Vertex* foundVertex { nullptr };
 		    if ( vertices.isValid() )
 		    {
-		      for ( const auto& vx : *vertices )
+		      for ( const auto vx : *vertices )
 		      {
 			for ( const auto& tpLink : vx->trackParticleLinks() )
 			{