tick: Move clocksource related stuff to timekeeping.h

Move clocksource related stuff to timekeeping.h and remove the
pointless include from ntp.c

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
[ rjw: Subject ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/2714218.nM5AEfAHj0@vostro.rjw.lan


Signed-off-by: Ingo Molnar <mingo@kernel.org>