TrigServices: Thread-safety cleanup.

Thread-safety annotations should match between declarations and definitions.

HLT/Trigger/TrigControl/TrigServices/src/TrigMonTHistSvc.h

 /*
-  Copyright (C) 2002-2019 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2020 CERN for the benefit of the ATLAS collaboration
 */
 
 #ifndef TRIGMONTHISTSVC_THISTSVC_H
@@ -7,6 +7,7 @@
 
 #include "GaudiKernel/ITHistSvc.h"
 #include "AthenaBaseComps/AthService.h"
+#include "CxxUtils/checker_macros.h"
 
 class TObject;
 class TH1;
@@ -44,7 +45,7 @@ class TrigMonTHistSvc: public extends<AthService, ITHistSvc>
 public:
   TrigMonTHistSvc(const std::string& name, ISvcLocator *svc );
 
-  virtual StatusCode initialize() override;
+  virtual StatusCode initialize ATLAS_NOT_THREAD_SAFE () override;
   virtual StatusCode stop() override;
   virtual StatusCode finalize() override;