panic: remove warning "'noreturn' function does return"

since commit

commit d2e8b911
Author: Mike Frysinger <vapier@gentoo.org>
Date:   Wed Jun 29 11:58:04 2011 +0000

    panic: add noreturn attribute

I see the following warnings:

vsprintf.c: In function 'panic':
vsprintf.c:730: warning: 'noreturn' function does return

for nearly all boards. This patch fixes this warning.

Signed-off-by: Heiko Schocher <hs@denx.de>
cc: Mike Frysinger <vapier@gentoo.org>