Add PSA_ERROR_STORAGE_FAILURE to psa_export_public_key

The same reason that it is included in psa_export_key
diff --git a/include/psa/crypto.h b/include/psa/crypto.h
index d570115..84f1646 100644
--- a/include/psa/crypto.h
+++ b/include/psa/crypto.h
@@ -868,6 +868,7 @@
  * \retval #PSA_ERROR_COMMUNICATION_FAILURE
  * \retval #PSA_ERROR_HARDWARE_FAILURE
  * \retval #PSA_ERROR_CORRUPTION_DETECTED
+ * \retval #PSA_ERROR_STORAGE_FAILURE
  * \retval #PSA_ERROR_INSUFFICIENT_MEMORY
  * \retval #PSA_ERROR_BAD_STATE
  *         The library has not been previously initialized by psa_crypto_init().