recent MFC of soreceive_dgram breaks kernels without INET6 option

[Josh Carroll]

> This was an oversight on my part -- none of our LINT build targets
> (apparently) excludes INET6.  It's fixed easily with an ifdef, and I've
> received re@ approval to merge the fix, so it's in the tree as of about an
> hour or two ago.  If you experience continuing problems after a cvsup,
> please let me know.

Thanks Robert! I pulled down the new version via cvsweb.freebsd.org,
since my cvsup mirror did not yet have the update. And the kernel is
building properly now.

Regards,
Josh