ox: have metadata node open

Should have been done alogn with e9f218cdf6.

Like this people who are not in the roster can get our public key and
write messages to use.

src/xmpp/ox.c

@@ -277,6 +277,12 @@ _ox_metadata_node__public_key(const char* const fingerprint)
     xmpp_stanza_add_child(pubsub, publish);
     xmpp_stanza_add_child(iq, pubsub);
 
+    if (connection_supports(XMPP_FEATURE_PUBSUB_PUBLISH_OPTIONS)) {
+        stanza_attach_publish_options(ctx, iq, "pubsub#access_model", "open");
+    } else {
+        log_debug("[OX] Cannot publish public key: no PUBSUB feature announced");
+    }
+
     iq_send_stanza(iq);
     xmpp_stanza_release(iq);
 }