AthExHive: Fix gcc9 warnings.

In C++11, implicit declarations of copy and assignment are deprecated if the
class has a user defined destructor or copy or assignment.  gcc9 now warns
about this by default.
Adjust to avoid the warning.

Control/AthenaExamples/AthExHive/AthExHive/HiveDataObj.h

 /*
-  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2018 CERN for the benefit of the ATLAS collaboration
 */
 
 #ifndef ATHEXHIVE_HIVEDATAOBJ_H
@@ -17,7 +17,7 @@ public:
  
   HiveDataObj():m_val(0) {};
   HiveDataObj(int i): m_val(i) {};
-  virtual ~HiveDataObj(){};
+  virtual ~HiveDataObj() = default;
 
   void val(int i) { m_val = i; }
   int val() const { return m_val; }