samba: add volatile file to support readonly rootfs

This patch adds a volatile file for samba which was removed by the
update from 3.6.25 to 4.1.12. This file is necessary to build a image
that uses the read-only-rootfs feature.

Signed-off-by: Johannes Pointner <johannes.pointner@br-automation.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>