fix imports

gc/happy/happy.lhs

@@ -25,6 +25,7 @@ Path settings auto-generated by Cabal:
 > import System.Console.GetOpt
 > import Control.Monad ( liftM )
 > import System.Environment
+> import System.Exit
 > import Data.Char
 > import System.IO
 > import Data.Array( assocs, elems, (!) )