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

We are going to split <linux/sched/task_stack.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/task_stack.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/arc/kernel/kgdb.c

@@ -10,6 +10,7 @@
 
 #include <linux/kgdb.h>
 #include <linux/sched.h>
+#include <linux/sched/task_stack.h>
 #include <asm/disasm.h>
 #include <asm/cacheflush.h>
 

arch/arc/kernel/process.c

@@ -12,6 +12,7 @@
 #include <linux/module.h>
 #include <linux/sched.h>
 #include <linux/sched/task.h>
+#include <linux/sched/task_stack.h>
 
 #include <linux/mm.h>
 #include <linux/fs.h>

arch/arc/kernel/ptrace.c

@@ -8,6 +8,7 @@
 
 #include <linux/ptrace.h>
 #include <linux/tracehook.h>
+#include <linux/sched/task_stack.h>
 #include <linux/regset.h>
 #include <linux/unistd.h>
 #include <linux/elf.h>

arch/arc/kernel/signal.c

@@ -53,6 +53,8 @@
 #include <linux/uaccess.h>
 #include <linux/syscalls.h>
 #include <linux/tracehook.h>
+#include <linux/sched/task_stack.h>
+
 #include <asm/ucontext.h>
 
 struct rt_sigframe {