Don't include <stddef.h> "For offsetof". This is not even wrong in
-current, since offsetof() is defined a header under /sys so that
system sources don't need to have this wrong include.

This bug was only detected because my version of <stddef.h> has some
spelling fixes (s/field/member/g) and gcc is now sensitive to the spelling
of arg names in macros as required by standards (ISO C90 6.8.3...).