s3-libnet: Fix format-nonliteral warning

BUG: https://bugzilla.samba.org/show_bug.cgi?id=12168

Signed-off-by: Amitay Isaacs <amitay@gmail.com>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
