xdrgen: Fix union declarations

Add a missing template file. This file is used when a union is
defined as a public API (ie, "pragma public <union name>;").

Signed-off-by: Chuck Lever <[e-mail]>

tools/net/sunrpc/xdrgen/templates/C/union/declaration/close.j2

···
+{# SPDX-License-Identifier: GPL-2.0 #}
+
+bool xdrgen_decode_{{ name }}(struct xdr_stream *xdr, struct {{ name }} *ptr);
+bool xdrgen_encode_{{ name }}(struct xdr_stream *xdr, const struct {{ name }} *value);