Give LocalADOLCStiffness a virtual constructor

I has virtual methods, and therefore also needs a virtual destructor.

dune/elasticity/assemblers/localadolcstiffness.hh

@@ -35,6 +35,8 @@ public:
     , useHessian2_(useHessian2)
     {}
 
+    virtual ~LocalADOLCStiffness() {}
+
     /** \brief Compute the energy at the current configuration */
     virtual double energy (const LocalView& localView,
                            const std::vector<double>& localConfiguration) const;