Update for changes in GHC API from backend type refactor

On the GHC repo, branch origin/wip/backend-as-record changes the names of the back ends in the GHC API. This change in Haddock is needed to make it compatible with the API changes. The MR with the GHC API change is ghc!7442 (merged)

haddock-api/src/Haddock.hs

@@ -542,7 +542,7 @@ withGhc' libDir needHieFiles flags ghcActs = runGhc (Just libDir) $ do
       let extra_opts | needHieFiles = [Opt_WriteHie, Opt_Haddock]
                      | otherwise = [Opt_Haddock]
           dynflags' = (foldl' gopt_set dynflags extra_opts)
-                        { backend = NoBackend
+                        { backend = noBackend
                         , ghcMode = CompManager
                         , ghcLink = NoLink
                         }