pfctl: Fix recursive printing of anchor labels

We recently noticed that the recursive printing of labels wasn't working
like the recursive printing of rules.

When running pfctl -sr -a* we get a listing of all rules, including the
ones inside anchors. On the other hand, when running pfctl -sl -a*, it
would only print the labels in the root level, just like without the
-a* argument.

As in our use-case we are interested on labels only and our labels are
unique even between anchors, we didn't add indentation or hierarchy to
the printing.

Sponsored by: InnoGames GmbH
Differential Revision: https://reviews.freebsd.org/D42728