<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.
