[ALSA] aoa: fix a sparse warning

This fixes a warning sparse gives.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>
diff --git a/sound/aoa/soundbus/soundbus.h b/sound/aoa/soundbus/soundbus.h
index 5c27297..622cd37 100644
--- a/sound/aoa/soundbus/soundbus.h
+++ b/sound/aoa/soundbus/soundbus.h
@@ -199,4 +199,6 @@
 extern int soundbus_register_driver(struct soundbus_driver *drv);
 extern void soundbus_unregister_driver(struct soundbus_driver *drv);
 
+extern struct device_attribute soundbus_dev_attrs[];
+
 #endif /* __SOUNDBUS_H */