ntp: security bump to version 4.2.8p5

Fixes:
CVE-2015-5300 - MITM attacker can force ntpd to make a step larger than
the panic threshold.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>