kernel - Revert "Fix buildworld."

* Fix PF in a different way, by conditionalizing the inclusion of
  struct pf_state instead of conditionalizing all the use cases for
  pfvar.h.

* This reverts commit 56e2aaa4d1de560d06f713866ab834747982f839.

* Reorders pfvar.h a bit and conditionalizes struct pf_state { }.