Fixed usage of stale define USE_SSL to ENABLE_SSL

Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <516BCEA2.6000608@fox-it.com>
URL: http://article.gmane.org/gmane.network.openvpn.devel/7509
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 1d561d4eaebe8652768270b6373023177b8d706d)