sched/headers: Prepare to move _init() prototypes from <linux/sched.h> to <linux/sched/init.h>

But first introduce a trivial header and update usage sites.

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>

arch/s390/kernel/setup.c

@@ -19,6 +19,7 @@
 #include <linux/export.h>
 #include <linux/sched.h>
 #include <linux/sched/task.h>
+#include <linux/cpu.h>
 #include <linux/kernel.h>
 #include <linux/memblock.h>
 #include <linux/mm.h>