Add missing extension point register for various settings backends

Using GMemorySettingsBackend, GRegistryBackend, GNullSettingsBackend, and GNextstepSettingsBackend before any other GSettingsBackend that calls _g_io_modules_ensure_extension_points_registered would cause the following warning: "Tried to implement non-registered extension point gsettings-backend". This is due to a missing call to _g_io_modules_ensure_extension_points_registered() in the GMemorySettingsBackend, GRegistryBackend, GNullSettingsBackend, and GNextstepSettingsBackend type definition which registers the gsettings-backend extension point.