libxml2: Don't build Python support, even Linux
libxml2: Don't build Python support, even Linux

libxml2 Python support requires Python 2 develpment headers which we no
longer install in bootstrap. This fixed build failure on latest Ubuntu
18.04 LTS. All other platforms already has this support disable.

https://bugzilla.gnome.org/show_bug.cgi?id=796781

fluendo/cerbero,GStreamer/cerbero,centricular/cerbero,atsushieno/cerbero,nirbheek/cerbero,fluendo/cerbero,fluendo/cerbero,centricular/cerbero,fluendo/cerbero,nirbheek/cerbero,centricular/cerbero,GStreamer/cerbero,atsushieno/cerbero,GStreamer/cerbero,nirbheek/cerbero,atsushieno/cerbero,atsushieno/cerbero,nirbheek/cerbero,GStreamer/cerbero,centricular/cerbero,centricular/cerbero,atsushieno/cerbero,fluendo/cerbero,GStreamer/cerbero