fedora-pkgdb-collections: Fix init() debug message

There is no plugin name set in the GObject::init() function yet, which results in

08:05:40:825 GsPluginFedoraPkgdbCollections disabling '(null)' as we're not Fedora

message in the debug log. Let's use the 'itself' term instead, because the plugin is identified by the log domain in the message.