x86/head/64: Add missing __head annotation to startup_64_load_idt()

This function is currently only used in the head code and is only called
from startup_64_setup_env(). Although it would be inlined by the
compiler, it would be better to mark it as __head too in case it doesn't.

Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/efcc5b5e18af880e415d884e072bf651c1fa7c34.1689130310.git.houwenlong.hwl@antgroup.com