packages/fff: Use fff-gateway package when compiling layer3 firmware

fff-gateway is mandatory when compiling layer3 firmware.

Signed-off-by: Fabian Bläse <fabian@blaese.de>
Reviewed-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Robert Langhammer <rlanghammer@web.de>