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>
diff --git a/lib/zebra.h b/lib/zebra.h
index ffca7a8..780e12e 100644
--- a/lib/zebra.h
+++ b/lib/zebra.h
@@ -40,6 +40,7 @@
 #include <unistd.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <stddef.h>
 #include <ctype.h>
 #include <errno.h>
 #include <fcntl.h>
@@ -175,7 +176,6 @@
 #include <linux/netlink.h>
 #include <linux/rtnetlink.h>
 #include <linux/filter.h>
-#include <stddef.h>
 #else
 #define RT_TABLE_MAIN		0
 #endif /* HAVE_NETLINK */