ntp: Disable debugging by default

There is a problem in configure.ac file that whether or not
'--enable-debugging' is specified in configure cmdline, debugging
is always enabled.
We should disable ntp debugging by default.

Signed-off-by: Xufeng Zhang <xufeng.zhang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>