dev-haskell/singletons-1.0: Add keywords

This ebuild was committed without any keywords, because it depends on
ghc 7.8, which didn't have any keywords at the time. Now that ghc
7.8 has keywords, singletons can get them, too.
Merge pull request #360 from atlaua/pu

dev-haskell/singletons-1.0: Add keywords