syscall: add missing TIOCGSID for openbsd/386

The previous syscall constants regeneration on openbsd was conducted
with OpenBSD current 3 months ago and it missed updating openbsd/386.
This CL adds TIOCGSID for fixing the inconsistency between opensbd/amd64
and openbsd/386.

Update #7049

LGTM=iant
R=jsing, rsc, iant
CC=golang-codereviews
https://golang.org/cl/96960044

src/pkg/syscall/zerrors_openbsd_386.go

@@ -1228,6 +1228,7 @@ const (
 	TIOCGETD                          = 0x4004741a
 	TIOCGFLAGS                        = 0x4004745d
 	TIOCGPGRP                         = 0x40047477
+	TIOCGSID                          = 0x40047463
 	TIOCGTSTAMP                       = 0x400c745b
 	TIOCGWINSZ                        = 0x40087468
 	TIOCMBIC                          = 0x8004746b