luci-proto-ipv6: fix dependency for IPv6

Signed-off-by: Rosy Song <rosysong@rosinson.com>

protocols/luci-proto-ipv6/Makefile

@@ -7,7 +7,7 @@
 include $(TOPDIR)/rules.mk
 
 LUCI_TITLE:=Support for DHCPv6/6in4/6to4/6rd/DS-Lite
-LUCI_DEPENDS:=
+LUCI_DEPENDS:=@IPV6
 
 PKG_LICENSE:=Apache-2.0