Introduce MBEDTLS_PRIVATE macro.

Public structs members are considered private and should not
be used by users application.

MBEDTLS_PRIVATE(member) macro is intended to clearly indicate
which members are private.

Signed-off-by: Mateusz Starzyk <mateusz.starzyk@mobica.com>
diff --git a/tests/src/drivers/test_driver_aead.c b/tests/src/drivers/test_driver_aead.c
index 25396c9..a147163 100644
--- a/tests/src/drivers/test_driver_aead.c
+++ b/tests/src/drivers/test_driver_aead.c
@@ -23,6 +23,8 @@
 #include MBEDTLS_CONFIG_FILE
 #endif
 
+#include <test/helpers.h>
+
 #if defined(MBEDTLS_PSA_CRYPTO_DRIVERS) && defined(PSA_CRYPTO_DRIVER_TEST)
 #include "psa_crypto_aead.h"