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

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

arch/um/drivers/line.c

@@ -5,8 +5,9 @@
 
 #include <linux/irqreturn.h>
 #include <linux/kd.h>
-#include <linux/sched.h>
+#include <linux/sched/signal.h>
 #include <linux/slab.h>
+
 #include "chan.h"
 #include <irq_kern.h>
 #include <irq_user.h>

arch/um/kernel/reboot.c

@@ -3,7 +3,7 @@
  * Licensed under the GPL
  */
 
-#include <linux/sched.h>
+#include <linux/sched/signal.h>
 #include <linux/spinlock.h>
 #include <linux/slab.h>
 #include <linux/oom.h>

arch/um/kernel/skas/mmu.c

@@ -5,8 +5,9 @@
  */
 
 #include <linux/mm.h>
-#include <linux/sched.h>
+#include <linux/sched/signal.h>
 #include <linux/slab.h>
+
 #include <asm/pgalloc.h>
 #include <asm/pgtable.h>
 #include <asm/sections.h>

arch/um/kernel/tlb.c

@@ -5,7 +5,8 @@
 
 #include <linux/mm.h>
 #include <linux/module.h>
-#include <linux/sched.h>
+#include <linux/sched/signal.h>
+
 #include <asm/pgtable.h>
 #include <asm/tlbflush.h>
 #include <as-layout.h>

arch/um/kernel/trap.c

@@ -4,7 +4,7 @@
  */
 
 #include <linux/mm.h>
-#include <linux/sched.h>
+#include <linux/sched/signal.h>
 #include <linux/hardirq.h>
 #include <linux/module.h>
 #include <linux/uaccess.h>