weston-init: add the weston user to the wayland group

Add the weston user to the wayland group so all users accessing the
global weston socket in /run all share a group.

Signed-off-by: Randolph Sapp <rs@ti.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit 30198b36)
Signed-off-by: Steve Sakoman <steve@sakoman.com>