Give ObstacleRestrictor an empty virtual destructor

Otherwise, non-default destructors in derived class are not called
properly when handling a derived class through a base pointer.

dune/solvers/transferoperators/obstaclerestrictor.hh

@@ -28,6 +28,8 @@ public:
                           const MultigridTransfer<DiscFuncType>& transfer,
                           const Dune::BitSetVector<blocksize>& critical) = 0;
 
+    virtual ~ObstacleRestrictor() = default;
+
 };
 
 #endif