glib-compat: isolate g_date_time_format_iso8601 version-bypass

The solution was discussed with Markus Armbruster during the review:
https://patchew.org/QEMU/20220323155743.1585078-1-marcandre.lureau@redhat.com/20220323155743.1585078-14-marcandre.lureau@redhat.com/

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20220420132624.2439741-3-marcandre.lureau@redhat.com>
2 files changed