immortalwrt默认固件添加ipv6

immortalwrt默认固件不带ipv6，需要手动安装ipv6helper

引用：【教程三】软路由**上网,openclash,openwrt IPv6配置

# https://www.hughh.top
# https://github.com/Hugh357/hugh357.github.io
wget --no-check-certificate -O "/usr/bin/ipv6-helper" https://openwrt.cc/scripts/ipv6-helper.sh
chmod +x /usr/bin/ipv6-helper
ipv6-helper install
# 最后重启OpenWrt