sched/headers: Prepare for new header dependencies before moving code to <linux/sched/nohz.h>

We are going to split <linux/sched/nohz.h> out of <linux/sched.h>, which
will have to be picked up from other headers and a couple of .c files.

Create a trivial placeholder <linux/sched/nohz.h> file that just
maps to <linux/sched.h> to make this patch obviously correct and
bisectable.

Include the new header in the files that are going to need it.
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>

include/linux/sched/nohz.h

+#ifndef _LINUX_SCHED_NOHZ_H
+#define _LINUX_SCHED_NOHZ_H
+
+#include <linux/sched.h>
+
+#endif /* _LINUX_SCHED_NOHZ_H */

kernel/sched/sched.h

@@ -10,6 +10,7 @@
 #include <linux/sched/mm.h>
 #include <linux/sched/cpufreq.h>
 #include <linux/sched/stat.h>
+#include <linux/sched/nohz.h>
 #include <linux/u64_stats_sync.h>
 #include <linux/sched/deadline.h>
 #include <linux/kernel_stat.h>

kernel/time/hrtimer.c

@@ -47,6 +47,7 @@
 #include <linux/sched/sysctl.h>
 #include <linux/sched/rt.h>
 #include <linux/sched/deadline.h>
+#include <linux/sched/nohz.h>
 #include <linux/timer.h>
 #include <linux/freezer.h>
 

kernel/time/tick-sched.c

@@ -22,6 +22,7 @@
 #include <linux/sched/signal.h>
 #include <linux/sched/clock.h>
 #include <linux/sched/stat.h>
+#include <linux/sched/nohz.h>
 #include <linux/module.h>
 #include <linux/irq_work.h>
 #include <linux/posix-timers.h>

kernel/time/timer.c

@@ -40,6 +40,7 @@
 #include <linux/irq_work.h>
 #include <linux/sched/signal.h>
 #include <linux/sched/sysctl.h>
+#include <linux/sched/nohz.h>
 #include <linux/slab.h>
 #include <linux/compat.h>