sched/headers: Remove <linux/signal.h> from <linux/sched.h>

Instead of including the full <linux/signal.h>, only include the types-only
<linux/signal_types.h> header in <linux/sched.h>, to further decouple the
scheduler header from the signal headers.
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>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/sched.h

@@ -11,7 +11,6 @@
 
 #include <linux/sem.h>
 #include <linux/shm.h>
-#include <linux/signal.h>
 #include <linux/signal_types.h>
 #include <linux/pid.h>
 #include <linux/seccomp.h>