[Qt] Fix libxml2 config test
https://bugs.webkit.org/show_bug.cgi?id=106317

Patch by Heikki Paajanen <heikki.paajanen@palm.com> on 2013-01-08
Reviewed by Simon Hausmann.

Don't assume standard include/libxml2, libxml-2.0.pc should know
where it's headers are.

* qmake/config.tests/libxml2/libxml2.cpp: Fix include

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@139052 268f45cc-cd09-0410-ab3c-d52691b4dbfc