Update openvpn-plugin.h for PolarSSL 1.3.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <5354F0F2.5080200@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/8567
Signed-off-by: Gert Doering <gert@greenie.muc.de>