lldpd: disable xml explicitly

This prevents auto-detection of libxml2 and thus the error:
Package lldpd is missing dependencies for the following libraries:
libxml2.so.2

Preventing a dependency to libxml2 is preferred, since libxml2
would be a out-of-(core-)tree dependency.

Reported-by: Buildbot
Signed-off-by: Michael Heimpold <mhei@heimpold.de>

SVN-Revision: 45859