Fix devicelist subscription and handle pubsub event

Devicelist subscription can be done directly with caps_add feature.

src/xmpp/omemo.h

@@ -1,3 +1,5 @@
+#include <glib.h>
+
 void omemo_devicelist_subscribe(void);
-void omemo_devicelist_publish(void);
+void omemo_devicelist_publish(GList *device_list);
 void omemo_bundle_publish(void);