task: Use GBinding as the task object in spawn_blocking

It's just a placeholder, but using cancellable is confusing, since it's
an actual type that GTask supports.

Author: 3v1n0

gio/src/task.rs

@@ -519,10 +519,10 @@ where
     T: Send + 'static,
     F: FnOnce() -> T + Send + 'static,
 {
-    // use Cancellable::NONE as source obj to fulfill `Send` requirement
-    let task = unsafe { Task::<bool>::new(Cancellable::NONE, Cancellable::NONE, |_, _| {}) };
+    // use a None GBinding as source obj to fulfill `Send` requirement
+    let task = unsafe { Task::<bool>::new(None::<&glib::Binding>, Cancellable::NONE, |_, _| {}) };
     let (join, tx) = JoinHandle::new();
-    task.run_in_thread(move |task, _: Option<&Cancellable>, _| {
+    task.run_in_thread(move |task, _: Option<&glib::Binding>, _| {
         let res = panic::catch_unwind(panic::AssertUnwindSafe(func));
         let _ = tx.send(res);
         unsafe { ffi::g_task_return_pointer(task.to_glib_none().0, ptr::null_mut(), None) }