gnome.iagno: add darwin support

pkgs/desktops/gnome/games/iagno/default.nix

···
     description = "Computer version of the game Reversi, more popularly called Othello";
     maintainers = teams.gnome.members;
     license = licenses.gpl3Plus;
-    platforms = platforms.linux;
+    platforms = platforms.unix;
   };
 }