weston-init: add weston user to the render group

The weston user must be in the render group in order to access render
device nodes for standard user-space graphics.

Signed-off-by: Randolph Sapp <rs@ti.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit 1cba8aa3)
Signed-off-by: Steve Sakoman <steve@sakoman.com>