chrony: enable pps support

Chrony has the ability to support PPS. Its enablement is determined
primarily from the existence of `sys/timepps.h`. This file is provided
from the pps-tools package. Adjusting Chrony's Makefile to depend on the
pps-tools package if a developer has included pps-tools as part of a
build. Relates to 2cfc966f.

Signed-off-by: James Knight <james.knight@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>