gnrc_ipv6_nib: always configure 802.15.4 long address

Our `gnrc_minimal` example configures the link-local address from the
IEEE 802.15.4 short address since it does not include 6Lo-ND.
This causes the application to be incompatible with our other GNRC
application that do include 6Lo-ND, since it [assumes][1] the link-local
address to be based on the EUI-64 for address resolution.

This enforces long addresses (aka EUI-64) for all IEEE 802.15.4 devices
when IPv6 is compiled in so `gnrc_minimal` is compatible again to the
rest.

Fixes #9910

[1]: https://tools.ietf.org/html/rfc6775#section-5.2

sys/net/gnrc/network_layer/ipv6/nib/nib.c

@@ -112,6 +112,7 @@ void gnrc_ipv6_nib_init_iface(gnrc_netif_t *netif)
     _init_iface_router(netif);
 #if GNRC_IPV6_NIB_CONF_6LN
     netif->ipv6.rs_sent = 0;
+#endif  /* GNRC_IPV6_NIB_CONF_6LN */
     if (netif->device_type == NETDEV_TYPE_IEEE802154) {
         /* see https://tools.ietf.org/html/rfc6775#section-5.2 */
         uint16_t src_len = IEEE802154_LONG_ADDRESS_LEN;
@@ -123,7 +124,6 @@ void gnrc_ipv6_nib_init_iface(gnrc_netif_t *netif)
          * directly everything else would deadlock anyway */
         netif->ops->set(netif, &opt);
     }
-#endif  /* GNRC_IPV6_NIB_CONF_6LN */
     netif->ipv6.na_sent = 0;
     if (gnrc_netif_ipv6_group_join_internal(netif,
                                             &ipv6_addr_all_nodes_link_local) < 0) {