x86, espfix: Fix broken header guard

Header guard is #ifndef, not #ifdef...
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

(cherry picked from commit 20b68535)
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>

arch/x86/include/asm/espfix.h

-#ifdef _ASM_X86_ESPFIX_H
+#ifndef _ASM_X86_ESPFIX_H
 #define _ASM_X86_ESPFIX_H
 
 #ifdef CONFIG_X86_64