MFC r303160.

Update leap-seconds to leap-seconds.3676752000.

As per https://datacenter.iers.org/web/guest/eop/-/somos/5Rgv/latest/16:

      UTC TIME STEP
on the 1st of January 2017

A positive leap second will be introduced at the end of December 2016.
The sequence of dates of the UTC second markers will be:

2016 December 31, 23h 59m 59s
2016 December 31, 23h 59m 60s
2017 January   1,  0h  0m  0s

The difference between UTC and the International Atomic Time TAI is:

from 2015 July 1, 0h UTC, to 2017 January 1 0h UTC   : UTC-TAI = - 36s
from 2017 January 1, 0h UTC, until further notice    : UTC-TAI = - 37s

Obtained from: ftp://tycho.usno.navy.mil/pub/ntp/leap-seconds.3676752000
See also: https://www.iers.org/SharedDocs/News/EN/BulletinC.html
https://datacenter.iers.org/web/guest/eop/-/somos/5Rgv/latest/16

Relnotes: yes