net: ipv6: Fix compile error if neighbor cache is disabled

If net-shell is enabled then it could try to access neighbor state
names. This would cause compile error as net_ipv6_nbr_state2str()
function is then missing.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>

subsys/net/ip/ipv6.c

@@ -590,6 +590,11 @@ struct in6_addr *net_ipv6_nbr_lookup_by_index(struct net_if *iface,
 
 	return NULL;
 }
+#else
+const char *net_ipv6_nbr_state2str(enum net_ipv6_nbr_state state)
+{
+	return "<unknown state>";
+}
 #endif /* CONFIG_NET_IPV6_NBR_CACHE */
 
 int net_ipv6_find_last_ext_hdr(struct net_pkt *pkt, u16_t *next_hdr_idx,