psa: Don't duplicate policy initializer

Use the PSA_KEY_POLICY_INIT macro in the definition of
PSA_CORE_KEY_ATTRIBUTES_INIT in order to avoid duplicating the key
policy initializer.
diff --git a/include/psa/crypto_struct.h b/include/psa/crypto_struct.h
index e28b6da..b1db423 100644
--- a/include/psa/crypto_struct.h
+++ b/include/psa/crypto_struct.h
@@ -379,7 +379,7 @@
     psa_key_attributes_flag_t flags;
 } psa_core_key_attributes_t;
 
-#define PSA_CORE_KEY_ATTRIBUTES_INIT {0, 0, 0, {0, 0, 0}, 0, 0}
+#define PSA_CORE_KEY_ATTRIBUTES_INIT {0, 0, 0, PSA_KEY_POLICY_INIT, 0, 0}
 
 struct psa_key_attributes_s
 {