50-udev-default.rules.in: set correct group for mediaX/cecX (#5921)

50-udev-default.rules.in: set correct group for mediaX/cecX (#5921)

The /dev/mediaX and /dev/cecX devices belong to the video group.
Add two default rules for that.

The /dev/cecX devices were introduced in kernel 4.8 in staging and moved
out of staging in 4.10. These devices support the HDMI CEC bus.

The /dev/mediaX devices are much older, but because they are not used very
frequently nobody got around to adding this rule to systemd. They let the
user control complex media pipelines.