move struct pf_state from pfvar.h to pfvar_priv.h.

move struct pf_state from pfvar.h to pfvar_priv.h.

we (sashan) are going to add a mutex to the pf_state struct, but a
mutex is a kernel data structure that changes shape depending on
things like whether MULTIPROCESSOR is enabled, and should therefore
not be visible to userland. when we added a mutex to pf_state,
compiling pfctl failed because it doesn't know what a mutex is and
it can't know which version of it the current kernel is running
with.

moving struct pf_state to pfvar_priv.h makes it clear it is a private
kernel only data structure, and avoids this leak into userland.

tested by me and make build
ok sashan@