uml: fix tty-related build error

/home/wangcong/Projects/linux-2.6/arch/um/drivers/line.c: In function `line_write_interrupt':
/home/wangcong/Projects/linux-2.6/arch/um/drivers/line.c:366: error: `struct tty_ldisc' has no member named `write_wakeup'
/home/wangcong/Projects/linux-2.6/arch/um/drivers/line.c:367: error: `struct tty_ldisc' has no member named `write_wakeup'

Signed-off-by: WANG Cong <wangcong@zeuux.org>
Cc: Jeff Dike <jdike@addtoit.com>
Acked-by: Alan Cox <alan@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>