[x86 setup] Fix typo in arch/i386/boot/header.S

There's an obvious typo in arch/i386/boot/header.S (in your
linux-2.6-x86setup.git) that I noticed by just studying the code.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>