nixos/mpd: Fix creation of "mpd" group.

The group is specified using a singleton list, so the loaOf merging is
done by iterating through the list items with imap, so it enumerates
every element and sets that as the default "name" attribute.

From lib/types:143:

name = elem.name or "unnamed-${toString defIdx}.${toString elemIdx}";

So, people get groups like "unnamed-X.Y" instead of "mpd".

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Reported-by: devhell <"^"@regexmail.net>
Tested-by: devhell <"^"@regexmail.net>

nixos/modules/services/audio/mpd.nix

@@ -125,6 +125,7 @@ in {
     });
 
     users.extraGroups = optionalAttrs (cfg.group == "mpd") (singleton {
+      name = "mpd";
       gid = gid;
     });
   };