rPackages.unrtf: fix build error

    island = [ pkgs.gsl.dev ];

    svKomodo = [ pkgs.which ];

    ulid = [ pkgs.zlib.dev ];

    unrtf = with pkgs; [ xz.dev bzip2.dev zlib.dev icu.dev ];

    nat = [ pkgs.which ];

    nat_templatebrains = [ pkgs.which ];

    pbdZMQ = [ pkgs.zeromq ] ++ lib.optionals stdenv.isDarwin [ pkgs.darwin.binutils ];