Fix compile error with GHC 7.2.*

While the Safe Haskell extension was introduced in GHC 7.2, -XUnsafe
extension was introduced in 7.4.

http://www.haskell.org/ghc/docs/7.2.2/html/users_guide/safe-haskell.html#safe-flag-summary
http://www.haskell.org/ghc/docs/7.4.1/html/users_guide/safe-haskell.html#safe-flag-summary