streamsynchronizer: reset eos on STREAM_START

streamsynchronizer: reset eos on STREAM_START

self->eos was never reset after streamsynchronizer has sent EOS
(except on explicit flush or switching back to PAUSED).
As a result, synchronization was broken if new streams were pushed later
as gst_stream_synchronizer_wait() does not wait if self->eos is set.

Fix this by reseting self->eos on STREAM_START as that means a new
stream is being sent upstream and so a new EOS will follow later on.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4755>