package/ntp: use common sysv init install

Previously the sysv init script install was conditional based on ntpd
being selected, now that sntp also has an init script and could be
selected independent of ntpd, a common install is necessary.

Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>