firewall: add commented disable_ipv6 option to default config

SVN-Revision: 21505

package/firewall/files/firewall.config

@@ -3,6 +3,8 @@ config defaults
 	option input		ACCEPT
 	option output		ACCEPT 
 	option forward		REJECT
+# Uncomment this line to disable ipv6 rules
+#	option disable_ipv6	1
 
 config zone
 	option name		lan