x86/boot: Correct setup_header.start_sys name

It is called start_sys_seg elsewhere so rename it to that. It is an
obsolete field so we could just as well directly call it __u16 __pad...

No functional change.

Signed-off-by: Borislav Petkov <bp@suse.de>
Link: http://lkml.kernel.org/r/20170221183639.16554-1-bp@alien8.de


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>