pf: remove COMPAT_FREEBSD14 #ifdef from pfvar.h

When userspace includes pfvar.h it doesn't get the kernel's COMPAT_*
defines, so we end up not having required symbols in userspace. This
caused the libpfctl port to fail to build.

libpfctl will be updated to use the new netlink-based state export code
soon, which will also fix thix build issue.

Sponsored by: Rubicon Communications, LLC ("Netgate")