openvpn: fix build without POLARSSL_DEBUG_C

Backport of upstream master commit
b63f98633dbe2ca92cd43fc6f8597ab283a600bf.

Signed-off-by: Magnus Kroken <mkroken@gmail.com>

From b63f98633dbe2ca92cd43fc6f8597ab283a600bf Mon Sep 17 00:00:00 2001
From: Steffan Karger <steffan@karger.me>
Date: Tue, 14 Jun 2016 22:00:03 +0200
Subject: [PATCH] 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>
--- a/src/openvpn/ssl_polarssl.c
+++ b/src/openvpn/ssl_polarssl.c
@@ -747,7 +747,9 @@ void key_state_ssl_init(struct key_state
   if (polar_ok(ssl_init(ks_ssl->ctx)))
     {
       /* Initialise SSL context */
+      #ifdef POLARSSL_DEBUG_C
       debug_set_threshold(3);
+      #endif
       ssl_set_dbg (ks_ssl->ctx, my_debug, NULL);
       ssl_set_endpoint (ks_ssl->ctx, ssl_ctx->endpoint);