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

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

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

Add a 'task_struct' predeclaration to make it build standalone.

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