ntp: fix hardcode to /usr/bin/perl

This patch properly uses the path variables and fixes ntptrace and
ntp-wait, just in case perl is not installed with the hardcode path.

Signed-off-by: Rahat Mahbub <rahat.mahbub@windriver.com>
Signed-off-by: Wenzong Fan <wenzong.fan@windriver.com>
Signed-off-by: Joe MacDonald <joe@deserted.net>