[Qt] Fix the build if libxslt is not available but libxml2 is
https://bugs.webkit.org/show_bug.cgi?id=106661

Reviewed by Simon Hausmann.

On Linux building the xml parser sources fails if the needed libxslt
dependencies are not installed but libxml2 is.

* WebCore.pri: add libxml2 to pkg-config if not on mac.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@139446 268f45cc-cd09-0410-ab3c-d52691b4dbfc