LocalDensity: Make destructor virtual

This is required in any class that has virtual methods.

dune/elasticity/densities/localdensity.hh

@@ -18,6 +18,9 @@ class LocalDensity
 
 public:
 
+  virtual ~LocalDensity()
+  {}
+
   /** \brief Evaluation with the deformation gradient
    *
    * \param x Position of the gradient