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

If we add <linux/uidgid.h> then <linux/sched/user.h> becomes a
self-contained header and users of it either don't need <linux/sched.h>
or have already included it.

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>

include/linux/sched/user.h

 #ifndef _LINUX_SCHED_USER_H
 #define _LINUX_SCHED_USER_H
 
-#include <linux/sched.h>
+#include <linux/uidgid.h>
+#include <linux/atomic.h>
 
 struct key;