win32: fix MLD for IPV6

The pcapif didn't set NETIF_FLAG_MLD6 which resulted in the solicit node
multicast group not being joined and thus no external host was able to
solicit our hardware address

This can be tested by pinging the win32 port from another host via IPv6

ports/win32/pcapif.c

@@ -1048,6 +1048,9 @@ pcapif_init(struct netif *netif)
 
   netif->mtu = 1500;
   netif->flags = NETIF_FLAG_BROADCAST | NETIF_FLAG_ETHARP | NETIF_FLAG_ETHERNET | NETIF_FLAG_IGMP;
+#if LWIP_IPV6 && LWIP_IPV6_MLD
+  netif->flags |= NETIF_FLAG_MLD6;
+#endif /* LWIP_IPV6 && LWIP_IPV6_MLD */
   netif->hwaddr_len = ETH_HWADDR_LEN;
 
   NETIF_INIT_SNMP(netif, snmp_ifType_ethernet_csmacd, 100000000);