Draft: postmarketos-ui-sxmo-de-sway: add user to feedbackd group

A new install of sxmo-de-sway on an a64 pinephone didn't let the LEDs blink. postmarketos-default-camera pulls in feedbackd, and feedbackd owns the LED devices, not the 'video' group, like sxmo expects. Adding the user to feedbackd gets the LEDsb linking again.

There may be a better way to do this. I just copied the post-install useradd from the phosh ui package. It's possible that feedbackd isn't present on every device?