Add missing MBEDTLS_PRIVATE

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
diff --git a/include/psa/crypto_struct.h b/include/psa/crypto_struct.h
index 4b1195d..2689415 100644
--- a/include/psa/crypto_struct.h
+++ b/include/psa/crypto_struct.h
@@ -157,7 +157,7 @@
      * ID values are auto-generated in psa_crypto_driver_wrappers.h
      * ID value zero means the context is not valid or not assigned to
      * any driver (i.e. none of the driver contexts are active). */
-    unsigned int id;
+    unsigned int MBEDTLS_PRIVATE(id);
 
     psa_algorithm_t MBEDTLS_PRIVATE(alg);
     psa_key_type_t MBEDTLS_PRIVATE(key_type);