Lexer.x: fix cyclic Functor instance

It looks like I introduced this cyclic definition in August 2014, but since
it isn't used, it hasn't been a problem...

src/compiler/GF/Grammar/Lexer.x

@@ -268,7 +268,7 @@ data ParseResult a
 newtype P a = P { unP :: AlexInput -> ParseResult a }
 
 instance Functor P where
-  fmap = (<$>)
+  fmap = liftA
 
 instance Applicative P where
   pure = return