Conditionally guard exit label to stop unused label error

Signed-off-by: Ryan Everett <ryan.everett@arm.com>
diff --git a/library/psa_crypto.c b/library/psa_crypto.c
index 3e7fc9a..cff9cca 100644
--- a/library/psa_crypto.c
+++ b/library/psa_crypto.c
@@ -1523,7 +1523,9 @@
                                            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);