AsgTools: Fix cppcheck warnings.

Don't let cppcheck see the #error directives.

Control/AthToolSupport/AsgTools/AsgTools/AsgComponent.h

 /*
-  Copyright (C) 2002-2020 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2021 CERN for the benefit of the ATLAS collaboration
 */
 
 /// \author Nils Krumnack
@@ -7,7 +7,7 @@
 #ifndef ASGTOOLS_ASGCOMPONENT_H
 #define ASGTOOLS_ASGCOMPONENT_H
 
-#ifndef XAOD_STANDALONE
+#if !defined(XAOD_STANDALONE) && !defined(__CPPCHECK__)
 #error "This header should only be used in XAOD_STANDALONE"
 #else
 

Control/AthToolSupport/AsgTools/AsgTools/SgTEvent.h

 /*
-  Copyright (C) 2002-2019 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2021 CERN for the benefit of the ATLAS collaboration
 */
 
 #ifndef ASGTOOLS_SGTEVENT_H
@@ -13,7 +13,7 @@
 #include "AsgMessaging/StatusCode.h"
 
 // Complain if we try using this header in Athena:
-#ifndef XAOD_STANDALONE
+#if !defined(XAOD_STANDALONE) && !defined(__CPPCHECK__)
 #   error( "This header should only be used for standalone ASG" )
 #endif
 

Control/AthToolSupport/AsgTools/AsgTools/SgTEventMeta.h

 /*
-  Copyright (C) 2002-2019 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2021 CERN for the benefit of the ATLAS collaboration
 */
 
 #ifndef ASGTOOLS_SGTEVENTMETA_H
@@ -9,7 +9,7 @@
 #include "AsgMessaging/StatusCode.h"
 
 // Complain if we try using this header in Athena:
-#ifndef XAOD_STANDALONE
+#if !defined(XAOD_STANDALONE) && !defined(__CPPCHECK__)
 #   error "This header should not be used in Athena"
 #endif // XAOD_STANDALONE