[functions.interfaces] add virtual dtor

dune/stuff/functions/interfaces.hh

@@ -538,6 +538,10 @@ private:
     {
     }
 
+    virtual ~Localfunction()
+    {
+    }
+
     virtual void evaluate(const DomainType& xx, RangeType& ret) const override final
     {
       const auto xx_global = geometry_.global(xx);