Fix build in the face of Applicative-Monad

Since 'empty' will now be exported by Control.Monad, this is needed to
disambiguate the 'empty' identifier from the one in Text.PrettyPrint.

Signed-off-by: Austin Seipp <aseipp@pobox.com>
Merge pull request #2021 from thoughtpolice/amp

Fix build in the face of Applicative-Monad