[bugfix] Fix makebasistest.cc
This was missing a header which was implicitly included when the MR was created. Now we can simply drop the VectorBackend, because interpolate will create and use an ISTLVectorBackend automatically.
This was missing a header which was implicitly included when the MR was created. Now we can simply drop the VectorBackend, because interpolate will create and use an ISTLVectorBackend automatically.