For unfathomable reasons, ipfilter abuses kernel data structures for its

For unfathomable reasons, ipfilter abuses kernel data structures for its
own purposes.  To pull this off, it defines _KERNEL before including the
headers where these structures are defined.  This leads to no end of
trouble when some of these headers, or other headers that they include,
change, as demonstrated by r180755.

The quick fix in this particular case is to define _WANT_FILE instead of
_KERNEL, conditional on __FreeBSD__.  A better long-term fix is left as
an exercise to the reader.