udisks2: Set G_VOLUME_IDENTIFIER_KIND_CLASS identifier for volumes

This identifier was discussed in

 https://bugzilla.gnome.org/show_bug.cgi?id=668295

and added to GLib with this commit

 http://git.gnome.org/browse/glib/commit/?id=10fbfcf09010e423cfc79c54e8da50b62a0f4b8a



For now, we do the ifndef dance instead of hard-requiring GLib master.

Signed-off-by: David Zeuthen <davidz@redhat.com>