Import qualified Prelude in Cmm/Parser.y

In preparation for the next version of 'happy', c95920 added a qualified
import to GHC/Parser.y but for some reason neglected GHC/Cmm/Parser.y

This patch adds the missing qualified import to GHC/Cmm/Parser.y and
also adds a clarifying comment to explain why this import is needed.

compiler/GHC/Cmm/Parser.y

@@ -203,6 +203,7 @@ necessary to the stack to accommodate it (e.g. 2).
 module GHC.Cmm.Parser ( parseCmmFile ) where
 
 import GHC.Prelude
+import qualified Prelude -- for happy-generated code
 
 import GHC.Platform
 import GHC.Platform.Profile

compiler/GHC/Parser.y

@@ -46,7 +46,7 @@ import Data.Char
 import Data.Maybe       ( maybeToList )
 import Control.Monad    ( mplus )
 import Control.Applicative ((<$))
-import qualified Prelude
+import qualified Prelude -- for happy-generated code
 
 -- compiler
 import GHC.Hs