ActsGeometry: Fix thread-safety checker warnings.

Fix some new thread-safety checker warnings. Missing const qualifier.

Tracking/Acts/ActsGeometry/ActsGeometry/ActsExtrapolationTool.h

 /*
-  Copyright (C) 2002-2021 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2022 CERN for the benefit of the ATLAS collaboration
 */
 
 #ifndef ACTSGEOMETRY_ACTSEXTRAPOLATIONTOOL_H
@@ -116,7 +116,7 @@ public:
 
 
 private:
-  std::unique_ptr<ActsExtrapolationDetail::VariantPropagator> m_varProp;
+  std::unique_ptr<const ActsExtrapolationDetail::VariantPropagator> m_varProp;
   std::unique_ptr<const Acts::Logger> m_logger{nullptr};
 
   SG::ReadCondHandleKey<AtlasFieldCacheCondObj> m_fieldCacheCondObjInputKey {this, "AtlasFieldCacheCondObj", "fieldCondObj", "Name of the Magnetic Field conditions object key"};