ssl_polarssl.c: fix includes and make casts explicit

ssl_polarssl.c: fix includes and make casts explicit

The master branch already has a commit doing almost the same
(9048d50), but since the API for polarssl 1.2 is different, this
could not be cherry-picked back to the 2.3 branch.

This commit:
 * adds a number of missing #includes.
 * makes a number of implicit casts explicit, to silence gcc
   -Wall and clang warnings that hide real problems.
 * changes the type of sha256_hash[] to match what polarssl expects.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1414230469-2670-1-git-send-email-steffan@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/9194
Signed-off-by: Gert Doering <gert@greenie.muc.de>