ECDH: Prevent direct access in non-legacy mode

Some sample programs access structure fields directly. Making these work is
desirable in the long term, but these are not essential for the core
functionality in non-legacy mode.

programs/test/benchmark.c

@@ -862,7 +862,7 @@ int main( int argc, char *argv[] )
     }
 #endif
 
-#if defined(MBEDTLS_ECDH_C)
+#if defined(MBEDTLS_ECDH_C) && defined(MBEDTLS_ECDH_LEGACY_CONTEXT)
     if( todo.ecdh )
     {
         mbedtls_ecdh_context ecdh;