haskell-ncurses: this package requires a pre 0.21 version of c2hs

Otherwise, the build fails, saying:

    c2hs: Errors during expansion of binding hooks:

    /nix/store/d6m9m2wqaymfiwbx11frdsm1cz7qjkdm-ncurses-5.9/include/curses.h:405: (column 2) [ERROR]  >>> Illegal type!
      The type specifiers of this declaration do not form a legal ANSI C(89) type.

This patch fixes https://github.com/NixOS/nixpkgs/issues/5873.

pkgs/development/haskell-modules/configuration-common.nix

@@ -317,6 +317,9 @@ self: super: {
   # https://github.com/athanclark/dag/issues/2
   dag = addBuildTool super.dag self.Cabal_1_22_0_0;     # http://hydra.cryp.to/build/498554/log/raw
 
+  # The build fails with the most recent version of c2hs.
+  ncurses = super.ncurses.override { c2hs = self.c2hs_0_20_1; };
+
 }
 // {
   # Not on Hackage yet.