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

We don't actually need the full rculist.h header anymore, include
the smaller rcupdate.h header instead.

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>