trafficshaper: avoid recursive dependency

trafficshaper: avoid recursive dependency

The conditional nftables dependency causes Kconfig to select
PACKAGE_nftables-nojson from itself:

  symbol PACKAGE_nftables-nojson is selected by PACKAGE_nftables-nojson

Split the firewall dependencies into nftables and iptables variants. Keep
trafficshaper as the default nftables variant and provide
trafficshaper-iptables for older systems.

Fixes: 2e945de232bd ("trafficshaper: add nftables firewall backend")
Signed-off-by: Dharmik Parmar <redacted>