sched/headers, time/timekeeping: Move the xtime_update() prototype from <linux/sched.h> to <linux/time.h>

This was in <linux/sched.h> only for hysterical raisins.
Acked-by: NLinus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: NIngo Molnar <mingo@kernel.org>