compositor: add option to enable weston_debug

Let users enable the compositor debug protocol on the compositor command
line. This allows weston-debug tool to work.

Signed-off-by: Pekka Paalanen <pq@iki.fi>
Signed-off-by: Maniraj Devadoss <Maniraj.Devadoss@in.bosch.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>