mbedtls: don't set debug threshold if compiled without MBEDTLS_DEBUG_C

mbedtls: don't set debug threshold if compiled without MBEDTLS_DEBUG_C

For targets with space constraints, one might want to compile mbed TLS
without MBEDTLS_DEBUG_C defined, to save some tens of kilobytes.  Make
sure OpenVPN still compiles if that is the case.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1465934403-22226-1-git-send-email-steffan@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/11922
Signed-off-by: Gert Doering <gert@greenie.muc.de>