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>

drivers/isdn/i4l/isdn_tty.c

@@ -15,6 +15,7 @@
 #include <linux/slab.h>
 #include <linux/delay.h>
 #include <linux/mutex.h>
+#include <linux/sched/signal.h>
 #include "isdn_common.h"
 #include "isdn_tty.h"
 #ifdef CONFIG_ISDN_AUDIO