selection.h: add tty_struct forward declaration

In file included from drivers/video/console/newport_con.c:16:
include/linux/selection.h:16: warning: "struct tty_struct" declared inside parameter list
include/linux/selection.h:16: warning: its scope is only this definition or declaration, which is probably not what you want

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>