Fix ssl_client2 and ssl_server2 if !PLATFORM_C

The CA callback changes introduce mbedtls_calloc() and
mbedtls_free() to ssl_client2 and ssl_server2, which
wasn't defined unless MBEDTLS_PLATFORM_C was set.

programs/ssl/ssl_server2.c

@@ -30,6 +30,7 @@
 #else
 #include <stdio.h>
 #include <stdlib.h>
+#define mbedtls_calloc     calloc
 #define mbedtls_free       free
 #define mbedtls_time       time
 #define mbedtls_time_t     time_t