Add ZendXML

This is required since libxml is not thread-safe and we've therefore some race conditions.
ZendXML avoids this by doing a string comparision when PHP-FPM is used.

Ref https://github.com/owncloud/core/issues/10427

@butonic As discussed.
@karlitschek Might deserve a backport. This is essential if we want to support PHP-FPM, currently there might be race-conditions which might either break your instance (unlikely) or make your instance vulnerable against some attack vectors (more likely)