Generate openvpn-plugin.h for MSVC build

Generate openvpn-plugin.h for MSVC build

openvpn-plugin.h was not generated for MSVC build since it has been
removed from sources and made generated by configure script.

This fix generates it for MSVC build and substitutes macroses like
@OPENVPN_VERSION_MAJOR@ with actual values.

Signed-off-by: Lev Stipakov <lstipakov@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1445500282-23129-1-git-send-email-lstipakov@gmail.com>
URL: http://article.gmane.org/gmane.network.openvpn.devel/10360
Signed-off-by: Gert Doering <gert@greenie.muc.de>