commonlib/helpers.h: Include stddef.h

The definition of offsetof() uses size_t, which is declared in stddef.h,
according to POSIX. Include stddef.h directly to avoid relying on
implicit inclusion.

Change-Id: I221be02c332de55c9fcf7d86673709ed43dd5c3c
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/22230
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Maximilian Schander <coreboot@mimoja.de>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/commonlib/include/commonlib/helpers.h

@@ -15,6 +15,10 @@
 #define COMMONLIB_HELPERS_H
 /* This file is for helpers for both coreboot firmware and its utilities. */
 
+#ifndef __ASSEMBLER__
+#include <stddef.h>
+#endif
+
 #ifndef ARRAY_SIZE
 #define ARRAY_SIZE(a) (sizeof(a) / sizeof((a)[0]))
 #endif