ostree: Use backward compatible smart pointer
ostree: Use backward compatible smart pointer

g_autoptr does not work in the Docker.nop11 CI container. This should
work and also be more consistent with the way other ostree pointers
are handled in this module.

Signed-off-by: Andy Doan <e3579b1e47f273529f0f929453e939a68ede9fd1@opensourcefoundries.com>

advancedtelematic/sota_client_cpp,advancedtelematic/aktualizr,advancedtelematic/aktualizr,advancedtelematic/aktualizr,advancedtelematic/aktualizr,advancedtelematic/sota_client_cpp