package/openssh: Set /var/empty permissions

package/openssh: Set /var/empty permissions

The openssh privilege separation feature, enabled by default,
requires that the path /var/empty exists and has certain permissions
(not writable by the sshd user). Note that nothing ever gets writting
in this directory, so it works fine on a readonly rootfs.

See README.privsep included as part of the openssh distribution.

Signed-off-by: Chris Lesiak <chris.lesiak@licor.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>