yash: fix build on darwin

pkgs/shells/yash/default.nix

···
 
   strictDeps = true;
   nativeBuildInputs = [ asciidoc gettext ];
-  buildInputs = [ ncurses ];
+  buildInputs = [ ncurses ] ++ lib.optionals stdenv.isDarwin [ gettext ];
 
   meta = with lib; {
     homepage = "https://yash.osdn.jp/index.html.en";