async: export current_is_async()

Introduced by 84b233adcca3 ("workqueue: implement current_is_async()").

Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

kernel/async.c

@@ -326,3 +326,4 @@ bool current_is_async(void)
 
 	return worker && worker->current_func == async_run_entry_fn;
 }
+EXPORT_SYMBOL_GPL(current_is_async);