[ExceptionHook] Fix "warning: 'struct Dune::ExceptionHook' has virtual

functions but non-virtual destructor"

[[Imported from SVN: r6251]]

dune/common/exceptions.hh

@@ -168,6 +168,7 @@ namespace Dune {
    */
   struct ExceptionHook
   {
+    virtual ~ExceptionHook() {}
     virtual void operator () () = 0;
   };