Fixup overlooked `unless` occurence

This was broken in eae19112462fe77a3f1298bff12b409b205a581d
Define fixity for `Data.Foldable.{elem,notElem}`

This was probably just an oversight. With this change the fixity matches
that from `Data.List.{elem,notElem`}`.

Addresses #9610

Reviewed By: austin, ekmett

Differential Revision: https://phabricator.haskell.org/D227
Merge branch 'master' of git://git.haskell.org/ghc into wip/T9596