drivers/char/nwbutton: Fix build breakage caused by include file reshuffling

Fix:

  drivers/char/nwbutton.c: In function 'button_sequence_finished':
  drivers/char/nwbutton.c:134:3: error: implicit declaration of function 'kill_cad_pid'

The declaration has been moved from one include file to another.
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: c3edc401 ("sched/headers: Move task_struct::signal and ...")
Link: http://lkml.kernel.org/r/1488762811-9022-1-git-send-email-linux@roeck-us.netSigned-off-by: Ingo Molnar <mingo@kernel.org>

drivers/char/nwbutton.c

@@ -6,7 +6,7 @@
 
 #include <linux/module.h>
 #include <linux/kernel.h>
-#include <linux/sched.h>
+#include <linux/sched/signal.h>
 #include <linux/interrupt.h>
 #include <linux/time.h>
 #include <linux/timer.h>