ntp: Cleanup timex.h

Move ntp_sycned to ntp.c and mark time_status as static.
Also yank function declaration for non-existant function.

CC: Thomas Gleixner <tglx@linutronix.de>
CC: Eric Dumazet <eric.dumazet@gmail.com>
CC: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>