[!353] Remove compiler warning when registering Richardson in factory.

Merge branch 'fix/warning-factory-richardson' into 'master'

ref:core/dune-istl This was suggested in [!349] but was already introduced in
master.

See merge request [core/dune-istl!353]

  [!349]: gitlab.dune-project.org/NoneNone/merge_requests/349
  [core/dune-istl!353]: gitlab.dune-project.org/core/dune-istl/merge_requests/353