gstreamer-1.0: Fix direction of Gst.Iterator.fold() ret parameter
I didn't catch this one while working on #1014 (closed). I found it just now while working on upstream https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/638 and am submitting this MR with the change!
Reference: in !159 (merged), we changed IteratorFoldFunction.ret
to ref
. Therefore, the uses of IteratorFoldFunction
should change to match. E.g., see the use here.