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/cbmem.h

@@ -3,7 +3,7 @@
 #ifndef _CBMEM_H_
 #define _CBMEM_H_
 
-#include <commonlib/bsd/cbmem_id.h>
+#include <commonlib/bsd/cbmem_id.h> /* IWYU pragma: export */
 #include <stddef.h>
 #include <stdint.h>
 #include <boot/coreboot_tables.h>