package/samba4: security bump to version 4.6.8

package/samba4: security bump to version 4.6.8

Release notes:
https://www.samba.org/samba/history/samba-4.6.8.html

Fixes
- CVE-2017-12150 (SMB1/2/3 connections may not require signing where
  they should)
- CVE-2017-12151 (SMB3 connections don't keep encryption across DFS
  redirects)
- CVE-2017-12163 (Server memory information leak over SMB1)

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>