Fix compile error in chromeos by adding stddef.h

Compile was failing with the following error:

In file included from src/vendorcode/google/chromeos/vboot_handoff.h:22:0,
                 from src/vendorcode/google/chromeos/chromeos.c:22:
vboot_reference/firmware/include/vboot_api.h:388:18: error: unknown type name 'size_t'
src/vendorcode/google/chromeos/chromeos.c: In function 'vboot_get_payload':
src/vendorcode/google/chromeos/chromeos.c:50:23: error: 'NULL' undeclared (first use in this function)

Change-Id: I13f9e41ef6a4151dc65a49eacfa0574083f72978
Signed-off-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/48289
Reviewed-on: http://review.coreboot.org/4131
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/vendorcode/google/chromeos/chromeos.c

@@ -17,6 +17,7 @@
  * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
+#include <stddef.h>
 #include "chromeos.h"
 #if CONFIG_VBOOT_VERIFY_FIRMWARE
 #include "vboot_handoff.h"