Fix kdump build when building it by hand.

I don't entirely like this approach, but it will only be temporarily,
namely until we get rid of COMPAT_43TTY. I do want <sys/ioctl_compat.h>
to cause a compiler error when included, because it's just there for
binary compatibility.

Reported by:	Andrzej Tobola <ato iem pw edu pl>

usr.bin/kdump/mkioctls

@@ -32,7 +32,7 @@ ioctl_includes=`
 `
 
 awk -v x="$ioctl_includes" 'BEGIN {print x}' |
-	gcc -E -I$1 -dM - |
+	gcc -E -I$1 -dM -DCOMPAT_43TTY - |
 	awk -v ioctl_includes="$ioctl_includes" -v use_switch="$use_switch" '
 BEGIN {
 	print "/* XXX obnoxious prerequisites. */"