MkIface: Ensure syntactic compatibility with ghc 8.0.1
MkIface: Ensure syntactic compatibility with ghc 8.0.1

Prior to 8.0.2 MultiWayIf syntax required that the -> token be indented relative
to the guard.

shlevy/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,shlevy/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc