[function] fix segfault in function test caused by non-virtual dtor

dune/stuff/function/interface.hh

@@ -48,6 +48,10 @@ public:
   typedef ThisType ComponentType;
   typedef Coefficient<RangeFieldType> CoefficientType;
 
+  virtual ~Interface()
+  {
+  }
+
   static const std::string id()
   {
     return "function";