ntp: security bump to version 4.2.8p3

Fixes:
CVE-2015-5146 - ntpd control message crash: Crafted NUL-byte in
configuration directive.

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