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

arch/x86/kernel/head64.c

@@ -588,7 +588,7 @@ static void set_bringup_idt_handler(gate_desc *idt, int n, void *handler)
 }
 
 /* This runs while still in the direct mapping */
-static void startup_64_load_idt(unsigned long physbase)
+static void __head startup_64_load_idt(unsigned long physbase)
 {
 	struct desc_ptr *desc = fixup_pointer(&bringup_idt_descr, physbase);
 	gate_desc *idt = fixup_pointer(bringup_idt_table, physbase);