package/ntp: fix installation

package/ntp: fix installation

Since 5d5c9a8 (ntp: security bump to version 4.2.8), scripts/ntp-wait
and scripts/ntptrace are no longer scripts themselves, but directories
containing those scripts.

Fixes:
    http://autobuild.buildroot.org/results/518/5189e84004bf28b891da3ee07fecc8717b4e8e8c/ (ntp-wait)
    http://autobuild.buildroot.org/results/005/0056192a06d44814a0279637c4bcbf602936e7ff/ (ntptrace)
    http://autobuild.buildroot.org/results/8f3/8f372acf73743edf8027cda6865ba1aa7b6413a3/ (both)
    ...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>