nfs-utils: make getnameinfo() required for --enable-gss

Systems that are so old that they don't have getnameinfo() in glibc are
probably also running kernels that are so old that they don't support
gssapi upcalls anyway.

Make --enable-gss dependent on the presence of the getnameinfo()
function.  This allows us to reduce some conditional compilation.

Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve Dickson <steved@redhat.com>