MFC r303978: Fix kernel build with TCP_RFC7413 option

The current in_pcb.h includes route.h, which includes sockaddr structures.
Including <sys/socketvar.h> should require <sys/socket.h>; add it in
the appropriate place.

PR: 211385
Submitted by: Sergey Kandaurov and iron at mail.ua
Reviewed by: gnn
Approved by: gnn (mentor)
Approved by: re (gjb)