package/samba4: security bump version to 4.18.6
Release notes: https://www.samba.org/samba/history/samba-4.18.6.html
Includes a mitigation for CVE-2007-4559.
Signed-off-by: Bernd Kuhls <bernd@kuhls.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>