Correct error message in mapDataTypes

dune/vtk/types.hh

@@ -108,7 +108,7 @@ namespace Dune
         case FLOAT32: caller(MetaType<float>{});         break;
         case FLOAT64: caller(MetaType<double>{});        break;
         default:
-          VTK_ASSERT_MSG(false, "Unsupported type " << to_string(t));
+          VTK_ASSERT_MSG(false, "Unsupported type " + to_string(t));
           break;
       }
     }