mbedtls: Fix debug function name

This patch is necessary so that curl compiles if MBEDTLS_DEBUG is
defined.

Bug: https://curl.haxx.se/mail/lib-2016-08/0001.html

lib/vtls/mbedtls.c

@@ -420,7 +420,7 @@ mbed_connect_step1(struct connectdata *conn,
 #endif
 
 #ifdef MBEDTLS_DEBUG
-  mbedtls_ssl_conf_dbg(&connssl->config, mbedtls_debug, data);
+  mbedtls_ssl_conf_dbg(&connssl->config, mbed_debug, data);
 #endif
 
   connssl->connecting_state = ssl_connect_2;