weston-init: make sure the render group exists

Add the render group explicitly here to make sure it exists for the
useradd command.

Signed-off-by: Randolph Sapp <rs@ti.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit 3134fca1)
Signed-off-by: Steve Sakoman <steve@sakoman.com>