bash-completion: fix syntax error

bash-completion: fix syntax error

There is a syntax error in a redirection:

  $ bash -x utils/ipset_bash_completion/ipset
  + shopt -s extglob
  utils/ipset_bash_completion/ipset: line 365: syntax error near unexpected token `('
  utils/ipset_bash_completion/ipset: line 365: `done < <(PATH=${PATH}:/sbin ( command ip -o link show ) )'

Move the environment variable assignment into the sub-shell.

Fixes: da6242e17583 ("Updated utilities")
Link: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1041605
Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>