workqueue: Make alloc/apply/free_workqueue_attrs() static

None of those functions have any users outside of workqueue.c. Confine
them.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Tejun Heo <tj@kernel.org>