Fix compilation with older mbed TLS versions (mbedtls_tls_prf_types undefined)

Fix compilation with older mbed TLS versions (mbedtls_tls_prf_types undefined)

The usage of the new keying material methods was not properly guarded.

To avoid a number of ifdefs this commit uses a dummy struct and function.
When we eventually drop support for non-EKM mbed TLS version we can remove
these.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20200825041647.26235-1-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg20812.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>