<stddef.h> is just fine and it doesn't break the use of libroot headers when compiling something with -nostdinc++ (in C++ mode). Not particularly important, but this gets compiling the libio C++ stuff when building a native gcc a bit further. It still fails, since our <printf.h> header is actually not usable (it includes <features.h>, which is not available in Haiku) -- something we should fix eventually.