system/dhcp6c: add NET_ICMPv6_AUTOCONF dependency

Previously the build failed if NET_ICMPv6_AUTOCONF was disabled
(due to the missing `netlib_obtain_ipv6addr` symbol).

Closes: apache#2412

system/dhcp6c/Kconfig

@@ -7,7 +7,7 @@ config SYSTEM_DHCPC_RENEW6
 	tristate "DHCP IPv6 Address Renewal"
 	default n
 	select NETUTILS_DHCP6C
-	depends on NET_UDP && NET_IPv6
+	depends on NET_UDP && NET_IPv6 && NET_ICMPv6_AUTOCONF
 	---help---
 		Enable the DHCP client 'renew6' command