zabbix: enable IPv6 support

zabbix: enable IPv6 support

compile tested on trunk, run tested on CC (ar71xx)

size before:
 96757 zabbix-agentd_2.4.6-1_ar71xx.ipk
 29180 zabbix-get_2.4.6-1_ar71xx.ipk
258979 zabbix-proxy_2.4.6-1_ar71xx.ipk
 35121 zabbix-sender_2.4.6-1_ar71xx.ipk
282885 zabbix-server_2.4.6-1_ar71xx.ipk

size after:
 97359 zabbix-agentd_2.4.6-2_ar71xx.ipk
 29841 zabbix-get_2.4.6-2_ar71xx.ipk
259948 zabbix-proxy_2.4.6-2_ar71xx.ipk
 35801 zabbix-sender_2.4.6-2_ar71xx.ipk
284019 zabbix-server_2.4.6-2_ar71xx.ipk

original PR by Thomas Bahn <thomas-bahn@gmx.net>

Signed-off-by: Etienne CHAMPETIER <champetier.etienne@gmail.com>