Explain structver usage in sample defer plugin.

Explain structver usage in sample defer plugin.

sample-plugins/defer/simple.c uses OPENVPN_PLUGINv3_STRUCTVER settings
that may not be obvious to a new author.  Add a comment to reduce
possible confusion.
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <1612163389-16421-1-git-send-email-gcox@mozilla.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg21540.html

Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit fdfbd4441c2225dc69431c57d18291e103c466cf)