TCP_PAWS_IDLE is does not exist in NetBSD and illumos. In FreeBSD
TCP_PAWS_IDLE is defined in netinet/tcp_seq.h, however this header
isn't included explicitly or implicitly at this point therefore
as far ipfilter is concerned TCP_PAWS_IDLE is not defined. Remove
the #ifdef and include netinet/tcp.h unconditionally.

MFC after: 1 week