ntp: import ntp-4.2.8p17

Fixes two small bugs including one regression.

Merge commit 'ab1f1aa8333369a83ff284848fc3fc2e52d5f29f'

(cherry picked from commit e6bfd18d21b225af6a0ed67ceeaf1293b7b9eba5)

ntp: Fix build

Two files used to obtain time from reference clocks did not include
ntp_types.h resulting in an undefined NONEMPTY_TRANSLATION_UNIT.

Fixes: e6bfd18d21b2

(cherry picked from commit 82aa1470c94c55db31c0168d65f8a08fa1d41378)