[spaces] fix clang compile

GCC, for some reason, is perfectly fine with this missing member,
although it's explicitly accessed in a test!

dune/gdt/spaces/cg/default.hh

@@ -83,6 +83,7 @@ public:
   using RangeFieldType = R;
   using BackendType = double;
   static const constexpr XT::Grid::Backends layer_backend = XT::Grid::Backends::view;
+  static const constexpr Backends backend_type{Backends::gdt};
   typedef DofCommunicationChooser<GridLayerType> DofCommunicationChooserType;
   typedef typename DofCommunicationChooserType::Type DofCommunicatorType;
 }; // class ContinuousLagrangeSpaceTraits

dune/gdt/spaces/rt/default.hh

@@ -86,6 +86,7 @@ public:
   using RangeFieldType = R;
   using BackendType = double;
   static const constexpr XT::Grid::Backends layer_backend = XT::Grid::Backends::view;
+  static const constexpr Backends backend_type{Backends::gdt};
   typedef DofCommunicationChooser<GridLayerType, false> DofCommunicationChooserType;
   typedef typename DofCommunicationChooserType::Type DofCommunicatorType;
 }; // class RtSpaceTraits