ntp: security bump to version 4.2.8p2

Fixes:
CVE-2015-1798 - ntpd accepts unauthenticated packets with symmetric key
crypto.
CVE-2015-1799 -  Authentication doesn't protect symmetric associations
against DoS attacks.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>