More explicit versioning compatibility in sample-plugins/defer/simple.c

More explicit versioning compatibility in sample-plugins/defer/simple.c

While not required, adding openvpn_plugin_min_version_required_v1 helps
by making an example for others to copy, and helps to explicitly call
attention to the difference between the API version number and the
struct version number in v3 calls.
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <1611778909-20630-2-git-send-email-gcox@mozilla.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg21508.html

Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit a385a3e8a28f2ce96c7ee0be8940b257765add5a)