dev-haskell/happy: drop depend on itself (noticed by gauthier)

Presence of .ly files led to cyclic depend:
> Building happy-1.19.0...
> Preprocessing executable 'happy' for happy-1.19.0...
> setup: The program happy is required but it could not be found

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>