Don't include net/if_var.h; it's not present on all *BSDs.

It also doesn't appear to be needed.  If a build fails on some version
of some BSD because it's not being included, *do not* add it back
unconditionally, add it back only for the BSD or BSDs on which it fails.
(NetBSD will not be one of those BSDs, as it doesn't *have* a
net/if_var.h header.)

capture/iface_monitor.c

@@ -374,7 +374,6 @@ iface_mon_event(void)
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <net/if.h>
-#include <net/if_var.h>
 #include <net/route.h>
 #include <net/if_dl.h>