workqueue: rename kernel/workqueue_sched.h to kernel/workqueue_internal.h

Workqueue wants to expose more interface internal to kernel/.  Instead
of adding a new header file, repurpose kernel/workqueue_sched.h.
Rename it to workqueue_internal.h and add include protector.

This patch doesn't introduce any functional changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>

kernel/workqueue.c

@@ -43,7 +43,7 @@
 #include <linux/idr.h>
 #include <linux/hashtable.h>
 
-#include "workqueue_sched.h"
+#include "workqueue_internal.h"
 
 enum {
 	/*