kthread: make __kthread_queue_delayed_work static

The __kthread_queue_delayed_work is not exported so
make it static, to avoid the following sparse warning:

  kernel/kthread.c:869:6: warning: symbol '__kthread_queue_delayed_work' was not declared. Should it be static?
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/kthread.c

@@ -866,7 +866,7 @@ void kthread_delayed_work_timer_fn(struct timer_list *t)
 }
 EXPORT_SYMBOL(kthread_delayed_work_timer_fn);
 
-void __kthread_queue_delayed_work(struct kthread_worker *worker,
+static void __kthread_queue_delayed_work(struct kthread_worker *worker,
 					 struct kthread_delayed_work *dwork,
 					 unsigned long delay)
 {