Fix compiler warnings in ssl_polarssl.c.

No functional changes, just add missing includes and make casts explicit.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1408396155-9017-1-git-send-email-steffan@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/8991
Signed-off-by: Gert Doering <gert@greenie.muc.de>