psa: Enable easier initialization of cipher operations

The struct psa_cipher_operation_s is built with a
mbedtls_cipher_context_t. The shape of mbedtls_cipher_context_t and an
initializer that works with Clang 5.0 and its
-Wmissing-field-initializers varies based on the configuration of the
library. Instead of making multiple initializers based on a maze of
ifdefs for all combinations of MBEDTLS_CIPHER_MODE_WITH_PADDING,
MBEDTLS_CMAC_C, and MBEDTLS_USE_PSA_CRYPTO, add a dummy variable to
psa_cipher_operation_s's union that encloses mbedtls_cipher_context_t.
This allows us to initialize the dummy with a Clang-approved initializer
and have it properly initialize the entire object.

include/psa/crypto_struct.h

@@ -140,6 +140,7 @@ struct psa_cipher_operation_s
     uint8_t block_size;
     union
     {
+        unsigned dummy; /* Enable easier initializing of the union. */
         mbedtls_cipher_context_t cipher;
     } ctx;
 };