It's not clear to me why this argument was excluded in the first place,
and Dan doesn't remember either. At least for consistency with
unescape_string, add it.
See also:
https://gitlab.gnome.org/GNOME/glib/-/merge_requests/1574#note_867283
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>