compat_ioctl: add compat handler for TIOCGSID ioctl

This is used by tcgetsid(3).

Signed-off-by: Andreas Schwab <schwab@linux-m68k.org>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>