CVE-2016-2110: auth/credentials: pass server_timestamp to cli_credentials_get_ntlm_response()

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11644

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Günther Deschner <gd@samba.org>
