tizen.conf: PREFERRED_PROVIDER for libxml2-python
tizen.conf: PREFERRED_PROVIDER for libxml2-python

Tizen does not have python-libxml2, so for the sake of consistency,
use libxml2 as provider of libxml2-python.

Change-Id: If3f1301d1cbd5981486f6ff63c8fb86484fd635a

TizenTeam/meta-tizen,TizenTeam/meta-tizen,TizenTeam/meta-tizen,TizenTeam/meta-tizen