firewall4: prefer over firewall as dependency

firewall4: prefer over firewall as dependency

When the virtual package "uci-firewall" is installed, the choice
between "firewall" and "firewall4" is arbitrary, sometimes resulting
in one, sometimes the other.

Set the default variant on "firewall4" to make it the preferred
package when installed as a dependency.

Link: https://forum.openwrt.org/t/owut-openwrt-upgrade-tool/200035/1126
Signed-off-by: Eric Fahlgren <ericfahlgren@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/22328
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>