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/include/cpu/cpu.h

@@ -3,7 +3,7 @@
 #ifndef CPU_CPU_H
 #define CPU_CPU_H
 
-#include <arch/cpu.h>
+#include <arch/cpu.h>  /* IWYU pragma: export */
 #include <stdint.h>
 
 void cpu_initialize(unsigned int cpu_index);

src/include/cpu/x86/msr.h

@@ -3,7 +3,7 @@
 #ifndef CPU_X86_MSR_H
 #define CPU_X86_MSR_H
 
-#include <cpu/x86/msr_access.h>
+#include <cpu/x86/msr_access.h> /* IWYU pragma: export */
 
 /* Intel SDM: Table 2-1
  * IA-32 architectural MSR: Extended Feature Enable Register