Split out libprestwoodiface into its own binary package

As required by Debian Policy §8.

Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: André Magalhães <andre.magalhaes@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D5498