TrigEFMissingET: Fix clang10 warning.

Add a default ctor to PufitGrid::Tower.
Avoids warnings provoked by having a user-defined assignment operator.

Trigger/TrigAlgorithms/TrigEFMissingET/TrigEFMissingET/PufitGrid.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
  */
 
 /******************************************************************************
@@ -48,6 +48,8 @@ namespace HLT { namespace MET {
            */
           Tower(const PufitGrid* parent, std::size_t index);
 
+          Tower(const Tower&) = default;
+
           /**
            * @brief Copy assignment operator
            *