Give LocalIntegralEnergy a virtual destructor

It has other virtual methods, and therefore needs a
virtual destructor, too.

dune/elasticity/materials/localintegralenergy.hh

@@ -31,6 +31,10 @@ public:
   : localDensity_(ld)
   {}
 
+  /** \brief Virtual destructor */
+  virtual ~LocalIntegralEnergy()
+  {}
+
   /** \brief Assemble the energy for a single element */
   field_type energy (const Entity& e,
                      const LocalFiniteElement& localFiniteElement,