package/samba4: security bump to version 4.9.6

package/samba4: security bump to version 4.9.6

Fixes the following security vulnerabilities:

 - CVE-2019-3870:
   During the provision of a new Active Directory DC, some files in the private/
   directory are created world-writable.
   https://www.samba.org/samba/security/CVE-2019-3870.html

 - CVE-2019-3880:
   Authenticated users with write permission can trigger a symlink traversal to
   write or detect files outside the Samba share.
   https://www.samba.org/samba/security/CVE-2019-3880.html

For more details, see the release notes:
https://www.samba.org/samba/history/samba-4.9.6.html

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>