Updates the linux build script
model: avoid repeat RutModelPrivate typedef

There was a typedef for RutModelPrivate in rut-model.c and rut-model.h
which would result in build failures.
rig-wrapper: resolve pkg_dir to absolute dir

This makes sure when running rig via rig-wrapper.sh that we resolve the
absolute path to rig used to create a rig.desktop file otherwise the
.desktop file won't work when installed.
build.sh: make sure to include libxml2 in the package

We had an issue with depending on the system's libxml2 so now bundle our
own