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>
diff --git a/init/main.c b/init/main.c
index ca9f53f..eae2f15 100644
--- a/init/main.c
+++ b/init/main.c
@@ -63,6 +63,7 @@
 #include <linux/device.h>
 #include <linux/kthread.h>
 #include <linux/sched.h>
+#include <linux/sched/init.h>
 #include <linux/signal.h>
 #include <linux/idr.h>
 #include <linux/kgdb.h>