commonlib/bsd, libpayload: Do not include helpers.h in stddef.h

`stddef.h` should only provide the definitions defined by ISO or Posix.
The included `commonlib/bsd/helpers.h` provide a lot of non standard
definitions that may interfere with definitions from the application.

Change-Id: Ia71edbc3ffe6694ff4b971decf3a41f915264bc8
Signed-off-by: Thomas Heijligen <src@posteo.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/70116
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

payloads/libpayload/include/stddef.h

@@ -2,7 +2,10 @@
 #define _STDDEF_H
 
 #include <arch/types.h>
-#include <commonlib/bsd/helpers.h>
+
+#if !defined(offsetof)
+#define offsetof(type, member) __builtin_offsetof(type, member)
+#endif
 
 #ifndef __WCHAR_TYPE__
 #define __WCHAR_TYPE__ int