sched/headers: Move the 'root_task_group' declaration to <linux/sched/autogroup.h>

Also remove the duplicate declaration from <linux/init_task.h>.

( That declaration was originally duplicated for dependency hell reasons,
  but there's no problem including the much smaller <linux/sched/autogroup.h>
  header now, to pick up the right prototype. )

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>