x86/head64: Add missing __head annotation to sme_postprocess_startup()

This function was previously part of __startup_64() which is marked
__head, and is currently only called from there. Mark it __head too.

Signed-off-by: Marco Bonelli <marco@mebeim.net>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220211162350.11780-1-marco@mebeim.net