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/pkey/ecdh_curve25519.c

@@ -35,12 +35,12 @@
 #define MBEDTLS_EXIT_FAILURE    EXIT_FAILURE
 #endif /* MBEDTLS_PLATFORM_C */
 
-#if !defined(MBEDTLS_ECDH_C) || \
+#if !defined(MBEDTLS_ECDH_C) || !defined(MBEDTLS_ECDH_LEGACY_CONTEXT) || \
     !defined(MBEDTLS_ECP_DP_CURVE25519_ENABLED) || \
     !defined(MBEDTLS_ENTROPY_C) || !defined(MBEDTLS_CTR_DRBG_C)
 int main( void )
 {
-    mbedtls_printf( "MBEDTLS_ECDH_C and/or "
+    mbedtls_printf( "MBEDTLS_ECDH_C and/or MBEDTLS_ECDH_LEGACY_CONTEXT and/or "
                     "MBEDTLS_ECP_DP_CURVE25519_ENABLED and/or "
                     "MBEDTLS_ENTROPY_C and/or MBEDTLS_CTR_DRBG_C "
                     "not defined\n" );

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;