Function gio_sys::g_proxy_connect_async

pub unsafe extern fn g_proxy_connect_async(proxy: *mut GProxy,
                                           connection: *mut GIOStream,
                                           proxy_address: *mut GProxyAddress,
                                           cancellable: *mut GCancellable,
                                           callback: GAsyncReadyCallback,
                                           user_data: gpointer)