mesa3d: requires host-libxml2 and host-python to generate some files

mesa3d now generates some C files at build time (related to the OpenGL
API) from XML files. This generation process is done using Python
scripts that require the libxml2 Python module.

Patch based on the initial work of Will Wagner (Thanks Will).

Signed-off-by: Vellemans Noel <noel.vellemans@visionbms.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>