MUC-PMs: add <x/> tag when sending them

A recent change to XEP-0045 §7.5 added this passage:

To allow for proper synchronization of these messages to the user's other clients by Message Carbons (XEP-0280) [19], the sending client SHOULD add an element qualified by the 'http://jabber.org/protocol/muc#user' namespace to the message.

Please implement this, so that other clients can properly detect Carbonated and MAMified PMs.