lib: unconditionally include stddef.h

lib: unconditionally include stddef.h

I've used offsetof() in the previous commit to paper over the security
problems in ospf_api.c.  This blows the build on FreeBSD 7.0, missing
offsetof().  Let's add that to zebra's generally used includes.

stddef.h (and offsetof) is defined in C89 section 4.1.5 (and not
deprecated/removed by any later standard).  If this causes problems, the
bug report should go against the host OS/compiler...

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>