tc: Add JSON output support to multiq

Since qdisc core already assumes all qdiscs are able to output JSON,
add JSON output support to multiq.

Fixes: c91d262f414d ("tc: jsonify qdisc core")
Signed-off-by: Victor Nogueira <victor@mojatatu.com>
Signed-off-by: David Ahern <dsahern@kernel.org>