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.