package/samba4: security bump version to 4.11.15

package/samba4: security bump version to 4.11.15

Fixes
o CVE-2020-14318: Missing handle permissions check in SMB1/2/3 ChangeNotify.
o CVE-2020-14323: Unprivileged user can crash winbind.
o CVE-2020-14383: An authenticated user can crash the DCE/RPC DNS with easily
                  crafted records.

Release notes:
https://www.samba.org/samba/history/samba-4.11.14.html (bugfix-only)
https://www.samba.org/samba/history/samba-4.11.15.html

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>