pmb.config: Add default user to the dialout group

This allows access to serial devices. My reason for wanting this is because Plasma Bigscreen uses HDMI CEC to allow remote control via the TV remote, and the CEC adapter on devices appear as /dev/ttyACM0, which is owned by the dialout group. Without adding the user to this group, the user can not control their device through HDMI CEC.

CC @craftyguy since it's useful for him on the L5 with gnss as well