Conditionally guard exit label to deter unused label error

Co-authored-by: David Horstmann <david.horstmann@arm.com>
Signed-off-by: Ryan Everett <ryan.everett@arm.com>
diff --git a/library/psa_crypto.c b/library/psa_crypto.c
index 4fc940c..da70db0 100644
--- a/library/psa_crypto.c
+++ b/library/psa_crypto.c
@@ -1639,7 +1639,9 @@
     status = psa_driver_wrapper_export_key(&attributes,
                                            slot->key.data, slot->key.bytes,
                                            data, data_size, data_length);
+#if defined(MBEDTLS_PSA_COPY_CALLER_BUFFERS)
 exit:
+#endif
     unlock_status = psa_unlock_key_slot(slot);
 
     LOCAL_OUTPUT_FREE(data_external, data);