openvpn: fix missing cipher list for polarssl in v2.3.11

Upstream OpenSSL hardening work introduced a change in shared code that
causes polarssl / mbedtls builds to break when no --tls-cipher is specified.

Import the upstream fix commit as patch until the next OpenVPN release gets
released and packaged.

Reported-by: Sebastian Koch <seb@metafly.info>
Signed-off-by: Jo-Philipp Wich <jo@mein.io>