polkit-group-rule: Add polkitd user and group

Recipes including polkit-group-rule.inc correctly install a directory
with the user and group set as 'polkitd'.  To avoid warnings like
these when assemblying the rootfs,

  WARNING: log_check: warning: user polkitd does not exist - using root
  ...
  WARNING: log_check: warning: group polkitd does not exist - using root

create this user and group.

Note: although the polkit recipe itself, on which this depends, is
      creating this same user and group, it seems that the useradd class
      needs this to be specified independently.

Signed-off-by: Ash Charles <ashcharles@gmail.com>
Acked-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>