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

We are going to split <linux/sched/numa_balancing.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/numa_balancing.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>

kernel/fork.c

@@ -16,6 +16,7 @@
 #include <linux/sched/mm.h>
 #include <linux/sched/coredump.h>
 #include <linux/sched/user.h>
+#include <linux/sched/numa_balancing.h>
 #include <linux/init.h>
 #include <linux/unistd.h>
 #include <linux/module.h>

kernel/sched/sched.h

@@ -6,6 +6,7 @@
 #include <linux/sched/clock.h>
 #include <linux/sched/wake_q.h>
 #include <linux/sched/signal.h>
+#include <linux/sched/numa_balancing.h>
 #include <linux/sched/mm.h>
 #include <linux/sched/cpufreq.h>
 #include <linux/u64_stats_sync.h>