gconf: Use a dev output

This reduces the closure size of gconf from 225 to 63 MiB.

pkgs/desktops/gnome-3/3.22/core/gconf/default.nix

···
 
   # ToDo: ldap reported as not found but afterwards reported as supported
 
+  outputs = [ "out" "dev" ];
+
   meta = with stdenv.lib; {
     homepage = http://projects.gnome.org/gconf/;
     description = "A system for storing application preferences";