Cannot enable IPv6 on BK72xx

[dwmw2]

I'm trying to build ESPHome with IPv6 support: esphome/feature-requests#2853

After fixing what seems to be a LegacyIPism in cores/beken-72xx/arduino/libraries/WiFi/WiFiGeneric.cpp it's still not building due to missing struct ipv6_config:

/home/dwmw/.platformio/packages/library-lwip/port/lwip_netif_address.h:41:9: note: forward declaration of 'struct ipv6_config'
   41 |  struct ipv6_config ipv6[MAX_IPV6_ADDRESSES];