ntp: add deterministic support for KPPS interface

ntp checks for presence of sys/timepps.h to determine whether the kernel
supports the RFC 2783 KPPS interface.  Under Linux the pps-tools package
installs this header.  Without this feature the ATOM clock driver does
not work, and other drivers like NMEA have reduced precision.  Remove
the feature non-determinism and increase ntpd capabilities by adding an
explicit dependency.

See: http://doc.ntp.org/4.2.6/kernpps.html



Signed-off-by: Peter A. Bigot <pab@pabigot.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>