ntp: add sntp.service for sntp package

This solves the following warning:

lib32-ntp-4.2.6p5: lib32-ntp: Files/directories were installed but not shipped
  /lib/systemd/system/sntp.service [installed-vs-shipped]

Signed-off-by: Chong Lu <Chong.Lu@windriver.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>