Added PSA_ERROR_INSUFFICIENT_MEMORY to psa_export_public_key

For the same reasons that psa_export_key can fail with this error
diff --git a/include/psa/crypto.h b/include/psa/crypto.h
index f787b13..8a987e9 100644
--- a/include/psa/crypto.h
+++ b/include/psa/crypto.h
@@ -687,6 +687,7 @@
  * \retval #PSA_ERROR_COMMUNICATION_FAILURE
  * \retval #PSA_ERROR_HARDWARE_FAILURE
  * \retval #PSA_ERROR_CORRUPTION_DETECTED
+ * \retval #PSA_ERROR_INSUFFICIENT_MEMORY
  * \retval #PSA_ERROR_BAD_STATE
  *         The library has not been previously initialized by psa_crypto_init().
  *         It is implementation-dependent whether a failure to initialize