libpfctl: fix label setting

A mismerge caused the labels list to be added to the wrong nvlist,
breaking label configuration.

If you compare the change from from main and stable/13 you
can see that main uses "nvl" and stable/13 has "nlvr" for
nvlist_append_string_array() but the backport changes it to "nlv".

This code was supposed to apply to pfctl_add_eth_rule() but instead
applied to pfctl_add_rule() for otherwise interesting reasons. Since
pfctl_add_eth_rule() uses "nvl" and pfctl_add_rule() uses "nvlr" but
also has "nvl" this compiled fine but still broke the label set.

Direct commit to stable/13.

PR: 275006