wait: allow to use __wake_up_pollfree() from GPL modules

commit ebafbcf7f32d ("UBUNTU: SAUCE: binder: turn into module")
is changing binder to be a module, but __wake_up_pollfree() can only be
used internally by the kernel.

Make __wake_up_pollfree an EXPORT_SYMBOL_GPL so that it can be used by
the binder module.

Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Signed-off-by: Alexandre Frade <kernel@xanmod.org>

kernel/sched/wait.c

@@ -258,6 +258,7 @@ void __wake_up_pollfree(struct wait_queue_head *wq_head)
 	/* POLLFREE must have cleared the queue. */
 	WARN_ON_ONCE(waitqueue_active(wq_head));
 }
+EXPORT_SYMBOL_GPL(__wake_up_pollfree);
 
 /*
  * Note: we use "set_current_state()" _after_ the wait-queue add,