TrkExUtils: Uninitialized class member.

ExtrapolationStep::time was not initialized in the constructor. Fixes gcc10 warning.

Tracking/TrkExtrapolation/TrkExUtils/TrkExUtils/ExtrapolationCell.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
 */
 
 ///////////////////////////////////////////////////////////////////
@@ -190,7 +190,8 @@ namespace Trk {
          materialPosition(Amg::Vector3D(0.,0.,0.)), 
          materialScaling(1.),         
          transportJacobian(tjac),
-         pathLength(pLength)
+         pathLength(pLength),
+         time(0)
         {}        
         
     };