Function gio_sys::g_file_append_to_async

pub unsafe extern fn g_file_append_to_async(file: *mut GFile,
                                            flags: GFileCreateFlags,
                                            io_priority: c_int,
                                            cancellable: *mut GCancellable,
                                            callback: GAsyncReadyCallback,
                                            user_data: gpointer)