ntp: security bump to version 4.2.8p6

CVE-2015-7973 - Deja Vu: Replay attack on authenticated broadcast mode
CVE-2015-7974 - Skeleton Key: Missing key check allows impersonation
between authenticated peers
CVE-2015-7975 - nextvar() missing length check
CVE-2015-7976 - ntpq saveconfig command allows dangerous characters in
filenames
CVE-2015-7977 - reslist NULL pointer dereference
CVE-2015-7978 - Stack exhaustion in recursive traversal of restriction
list
CVE-2015-7979 - Off-path Denial of Service (DoS) attack on authenticated
broadcast mode
CVE-2015-8137 - origin: Zero Origin Timestamp Bypass
CVE-2015-8158 - Potential Infinite Loop in ntpq

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