cc-wifi-connection-list: Use correct setter in property getter
CcWifiConnectionList::forgettable is of type `gboolean` so use
`g_value_set_boolean()` in the getter instead of `g_value_set_object()`.
CcWifiConnectionList::forgettable is of type `gboolean` so use
`g_value_set_boolean()` in the getter instead of `g_value_set_object()`.