Always process Neighbor Solicitation messages. This fixes IPv6

Neighbor Discovery not finding the ESP32 on Ethernet interfaces.

Author: ewpa

src/core/ipv6/ip6.c

@@ -476,7 +476,7 @@ ip6_input(struct pbuf *p, struct netif *inp)
     else if (mld6_lookfor_group(inp, ip6_current_dest_addr())) {
       netif = inp;
     }
-#else /* LWIP_IPV6_MLD */
+#endif /* LWIP_IPV6_MLD */
     else if (ip6_addr_issolicitednode(ip6_current_dest_addr())) {
       /* Filter solicited node packets when MLD is not enabled
        * (for Neighbor discovery). */
@@ -491,7 +491,6 @@ ip6_input(struct pbuf *p, struct netif *inp)
         }
       }
     }
-#endif /* LWIP_IPV6_MLD */
     else {
       netif = NULL;
     }