autoconf: Fix typo

In config.h, it would state:

      /* Enable systemd support */
      #define ENABLE_PLUGIN 1

instead of

      /* Enable plug-in support */
      #define ENABLE_PLUGIN 1

Signed-off-by: David Sommerseth <davids@redhat.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1370600130-6075-1-git-send-email-dazo@users.sourceforge.net>
URL: http://article.gmane.org/gmane.network.openvpn.devel/7658
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 8065cd1c65273ef05ba2ac66f15224e170a57290)