screamqueue: add missing clock unref
screamqueue: add missing clock unref

Closes #52.

sancane/openwebrtc-gst-plugins,EricssonResearch/openwebrtc-gst-plugins,sancane/openwebrtc-gst-plugins,EricssonResearch/openwebrtc-gst-plugins,sancane/openwebrtc-gst-plugins,EricssonResearch/openwebrtc-gst-plugins