Include <sys/cdefs.h>, <sys/featuretest.h>, and <sys/ansi.h>
unconditionally, not only #ifndef _KERNEL. The kernel declarations
require cdefs.h and standard practice is to include cdefs.h where it's
used; they also require sys/ansi.h; and while they don't require
featuretest.h it's also harmless... and includes should be at the top
anyhow.

PR 46557 from Richard Hansen.

(dholland)