netlink: specs: nftables: allow decode of tailscale ruleset

Fill another small gap in the nftables spec so that it is possible to
dump a tailscale ruleset with:

  tools/net/ynl/cli.py --spec \
     Documentation/netlink/specs/nftables.yaml --dump getrule

This adds support for the 'target' expression.

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20240904091024.3138-1-donald.hunter@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>