package/samba4: security bump to version 4.5.12

package/samba4: security bump to version 4.5.12

Fixes CVE-2017-11103:

All versions of Samba from 4.0.0 onwards using embedded Heimdal
Kerberos are vulnerable to a man-in-the-middle attack impersonating
a trusted server, who may gain elevated access to the domain by
returning malicious replication or authorization data.

Samba binaries built against MIT Kerberos are not vulnerable.

https://www.samba.org/samba/history/samba-4.5.12.html

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