Fix "Could not initialize class SmackConfiguration"

Fixes #1804. While developing for openhab, xpp3 slipped into the classpath but wasn't committed. Sorry for that.

Anyway, I'd like to develop further on the xmpp action. Since Smack now enforces SSL/TLS security, I'd like to add support for [java pinning](https://github.com/flowdalic/java-pinning) so that users can enjoy *secure* TLS connections. Should I base my work on the '1.6.1' or 'master' branch?