bin: Use from_glib_none() in Bin::add() for subclasses

This way we take ownership of any floating references passed in here, as
required by the GstBin API, instead of just borrowing the reference and
having someone else sink it or not.

gstreamer/src/subclass/bin.rs

@@ -116,7 +116,7 @@ where
     let wrap: Bin = from_glib_borrow(ptr);
 
     gst_panic_to_error!(&wrap, &instance.panicked(), false, {
-        match imp.add_element(&wrap, &from_glib_borrow(element)) {
+        match imp.add_element(&wrap, &from_glib_none(element)) {
             Ok(()) => true,
             Err(err) => {
                 err.log_with_object(&wrap);