openssh: security bump to version 7.8

Fixes CVE-2018-15473: user enumeration vulnerability due to not delaying
bailout for an invalid authenticating user until after the packet
containing the request has been fully parsed.

Some OpenSSH developers don't consider this a security issue:

  https://lists.mindrot.org/pipermail/openssh-unix-dev/2018-August/037138.html



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>