Guard the exit to stop unused label warning

Signed-off-by: Ryan Everett <ryan.everett@arm.com>
diff --git a/library/psa_crypto.c b/library/psa_crypto.c
index a09877e..e8ec3f2 100644
--- a/library/psa_crypto.c
+++ b/library/psa_crypto.c
@@ -7114,7 +7114,9 @@
     status = psa_key_derivation_input_internal(operation, step,
                                                PSA_KEY_TYPE_NONE,
                                                data, data_length);
+#if defined(MBEDTLS_PSA_COPY_CALLER_BUFFERS)
 exit:
+#endif
     LOCAL_INPUT_FREE(data_external, data);
     return status;
 }