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 <[e-mail]>
Cc: Mike Galbraith <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: [e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

include/linux/sched/autogroup.h

···
 #ifndef _LINUX_SCHED_AUTOGROUP_H
 #define _LINUX_SCHED_AUTOGROUP_H
 
-#include <linux/sched.h>
-
 struct signal_struct;
+struct task_struct;
 struct task_group;
 struct seq_file;