Add a <x muc#user> node on outgoing private MUC messages

See https://xmpp.org/extensions/xep-0045.html#privatemessage

fix #3321

src/bridge/bridge.cpp

@@ -862,7 +862,8 @@ void Bridge::send_message(const Iid& iid, const std::string& nick, const std::st
           const auto chan_name = Iid(Jid(it->second).local, {}).get_local();
           for (const auto& resource: this->resources_in_chan[ChannelKey{chan_name, iid.get_server()}])
             this->xmpp.send_message(it->second, this->make_xmpp_body(body, encoding),
-                                    this->user_jid + "/" + resource, "chat", true, true);
+                                    this->user_jid + "/"
+                                    + resource, "chat", true, true, true);
         }
       else
         {

src/xmpp/xmpp_component.cpp

@@ -269,7 +269,8 @@ void* XmppComponent::get_receive_buffer(const size_t size) const
 }
 
 void XmppComponent::send_message(const std::string& from, Xmpp::body&& body, const std::string& to,
-                                 const std::string& type, const bool fulljid, const bool nocopy)
+                                 const std::string& type, const bool fulljid, const bool nocopy,
+                                 const bool muc_private)
 {
   Stanza message("message");
   {
@@ -301,6 +302,11 @@ void XmppComponent::send_message(const std::string& from, Xmpp::body&& body, con
         XmlSubNode nocopy(message, "no-copy");
         nocopy["xmlns"] = "urn:xmpp:hints";
       }
+    if (muc_private)
+      {
+        XmlSubNode x(message, "x");
+        x["xmlns"] = MUC_USER_NS;
+      }
   }
   this->send_stanza(message);
 }

src/xmpp/xmpp_component.hpp

@@ -112,7 +112,8 @@ public:
    * server-part of the JID and must be added.
    */
   void send_message(const std::string& from, Xmpp::body&& body, const std::string& to,
-                    const std::string& type, const bool fulljid, const bool nocopy=false);
+                    const std::string& type, const bool fulljid, const bool nocopy=false,
+                    const bool muc_private=false);
   /**
    * Send a join from a new participant
    */