serial: fix compile warning about putc

drivers/serial/8250_early.c:80: warning: conflicting types for built-in function `putc'

Signed-off-by: Yinghai Lu <yinghai.lu@sun.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>