svcauth_unix.c: quiet sparse noise

Like svcauth_unix, the symbol svcauth_null is used external from this
file. Declare it as extern to quiet the following sparse noise:

warning: symbol 'svcauth_null' was not declared. Should it be static?

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: "J. Bruce Fields" <bfields@fieldses.org>
Cc: Neil Brown <neilb@suse.de>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>