fallbacksrc: send EOS on fallback-only stream
fallbacksrc: send EOS on fallback-only stream

When both audio and video are enabled, but the primary stream
only has either, when that stream ends we want to end the other
fallback-only stream as well.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/515>

sdroege/rsplugin,sdroege/rsplugin