Start tracking ntpsec as well for february's ntp security advisory

According to upstream's confirmation in

https://lists.ntpsec.org/pipermail/devel/2018-March/006008.html

only one of the CVEs from the ntp security advisory from

http://support.ntp.org/bin/view/Main/SecurityNotice#February_2018_ntp_4_2_8p11_NTP_S

is applicable.