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

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

( Keep the <linux/percpu.h> dependency.)

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>