Fix PrivateDataResult.getChildElementXML()

that method never worked correctly since 11 years, ie. the PrivateData
was never added as element text. It's not surprisingly that this was not
discovered in more then a decade, since Smack usually never *sends*
those stanza but only receives them.

But that's no reason to not fix it. :)

smack-extensions/src/main/java/org/jivesoftware/smackx/iqprivate/PrivateDataManager.java

@@ -285,7 +285,7 @@ public class PrivateDataManager extends Manager {
             StringBuilder buf = new StringBuilder();
             buf.append("<query xmlns=\"jabber:iq:private\">");
             if (privateData != null) {
-                privateData.toXML();
+                buf.append(privateData.toXML());
             }
             buf.append("</query>");
             return buf.toString();