chromeos: include stddef to fix compilation error

As some of the standard definitions were shuffled around
chromeos started failing to build. Correct this.

Change-Id: I9927441ccb2d646e8b3395e6e9f8e8166de74ab0
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/4844
Tested-by: build bot (Jenkins)
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>

src/vendorcode/google/chromeos/chromeos.h

@@ -20,6 +20,7 @@
 #ifndef __CHROMEOS_H__
 #define __CHROMEOS_H__
 
+#include <stddef.h>
 #include <stdint.h>
 
 /* functions implemented per mainboard: */