sched/headers: Remove <linux/sched.h> from <linux/sched/clock.h>

The <linux/sched/clock.h> file is a largely self-contained header and users of
it either don't need <linux/sched.h> - or have already included it.

This reduces the size of the header dependency graph.
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/sched/clock.h

 #ifndef _LINUX_SCHED_CLOCK_H
 #define _LINUX_SCHED_CLOCK_H
 
-#include <linux/sched.h>
+#include <linux/smp.h>
 
 /*
  * Do not use outside of architecture code which knows its limitations.