Add missing guard around a union field

Author: gilles-peskine-arm

library/psa_crypto_core.h

@@ -56,11 +56,13 @@ typedef struct
         /* EC public key or key pair */
         mbedtls_ecp_keypair *ecp;
 #endif /* MBEDTLS_ECP_C */
+#if defined(MBEDTLS_PSA_CRYPTO_SE_C)
         /* Any key type in a secure element */
         struct se
         {
             psa_key_slot_number_t slot_number;
         } se;
+#endif /* MBEDTLS_PSA_CRYPTO_SE_C */
     } data;
 } psa_key_slot_t;