AthLinks: Make DataProxyHolder::thin static.

So that it can be called directly from code implementing thinning in AthContainers. (In principle might be better to factor this out as a free function, but this is not really meant to be generally useful so that may be overkill.)

Control/AthLinks/AthLinks/tools/DataProxyHolder.h

 // This file's extension implies that it's C, but it's really -*- C++ -*-.
 
 /*
-  Copyright (C) 2002-2022 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2023 CERN for the benefit of the ATLAS collaboration
 */
 /**
  * @file AthLinks/tools/DataProxyHolder.h
@@ -359,6 +359,7 @@ public:
    *
    * Returns @c true if the index was changed; @c false otherwise.
    */
+  static
   bool thin (sgkey_t& sgkey, size_t& index,
              const SG::ThinningCache* thinningCache);