The OpenWrt function get_mac_label resides in /lib/functions/system.sh,
so it has to be included, if we want to use this function.
This include was missing in the fff-wireguard layer3 configuration
script, so it is added here.
Fixes: 68c7d75a1b ("treewide: exploit label MAC address from OpenWrt")
Signed-off-by: Fabian Bläse <fabian@blaese.de>