ntp: disable debugging

When running ntp it randomly aborts at ntp-4.2.8p8/libntp/recvbuff.c:326
which seems to be a debugging feature. This patch just disables
debugging, it does not fix the root cause of the problem.

Signed-off-by: Vicente Bergas <vicencb@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>