[Device] Free dlna class strings after use

Ownership of strings in the GList that
gupnp_device_info_list_dlna_device_class_identifier () returns is
fully transferred since GUPnP 0.20.4. Free the strings after use.

Fixes #129.

Signed-off-by: Jussi Kukkonen <jussi.kukkonen@intel.com>
Merge pull request #147 from jku/129

[Device] Free dlna class strings after use