includes: Add include guards to `stddef.h`

Change-Id: Ifae4f07abd75da9784967c2020eed2c3fe64afa0
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/55589
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

payloads/libpayload/include/stddef.h

@@ -1,3 +1,5 @@
+#ifndef _STDDEF_H
+#define _STDDEF_H
 
 #include <arch/types.h>
 
@@ -35,3 +37,5 @@ typedef __SIZE_TYPE__ ssize_t;
 #define NSECS_PER_MSEC (NSECS_PER_SEC / MSECS_PER_SEC)
 #define NSECS_PER_USEC (NSECS_PER_SEC / USECS_PER_SEC)
 #define USECS_PER_MSEC (USECS_PER_SEC / MSECS_PER_SEC)
+
+#endif