luci-mod-network: expose peerdns option for dhcpv6

Ref: https://github.com/openwrt/luci/commit/2be01cbf#commitcomment-111035308
Fixes: 2be01cbf

 ("luci-mod-network: restrict peerdns option to protocols that implemenent it")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>