treewide: Add 'IWYU pragma: export' comment

This pragma says to IWYU (Include What You Use) that the current file
is supposed to provide commented headers.

Change-Id: I482c645f6b5f955e532ad94def1b2f74f15ca908
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/68332
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Werner Zeh <werner.zeh@siemens.com>

src/arch/x86/include/arch/cpu.h

@@ -4,7 +4,7 @@
 #define ARCH_CPU_H
 
 #include <types.h>
-#include <arch/cpuid.h>
+#include <arch/cpuid.h> /* IWYU pragma: export */
 
 /*
  * EFLAGS bits