Add xml-conduit override to fix build

Author: maralorn

haskell-overlays/default.nix

@@ -117,6 +117,8 @@ rec {
 
   combined-ghc = self: super: foldExtensions [
     (self: super: {
+     # TODO: Upstream (to nixpkgs)
+     xml-conduit = haskellLib.dontCheck super.xml-conduit;
     })
     (optionalExtension (versionWildcard [ 8 6 ] super.ghc.version) ghc-8_6)
     (optionalExtension (lib.versionOlder "8.11" super.ghc.version) ghc-head)