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/string.h

@@ -3,9 +3,9 @@
 #ifndef STRING_H
 #define STRING_H
 
-#include <stdarg.h>
+#include <stdarg.h> /* IWYU pragma: export */
 #include <stddef.h>
-#include <stdio.h>
+#include <stdio.h> /* IWYU pragma: export */
 
 void *memcpy(void *dest, const void *src, size_t n);
 void *memmove(void *dest, const void *src, size_t n);