Update leap seconds file for NTP

Summary:
IETF doesn't publish leap seconds database anymore, IANA does.

As such, this fix allows to provision the next bulletin.

Test Plan: salt -G 'os:freebsd' state.apply hotfixes/leap-seconds

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D3289