rtspsrc: Chain up to the parent class' provide_clock() implementation
If no clock was provided directly by rtspsrc. This behaviour was removed by f8013487 and results in rtspsrc not providing the system clock via the rtpjitterbuffer. As a result, if another element like an audio sink, provides a clock, the pipeline would select that (when going to PAUSED/PLAYING again later). Audio clocks usually don't progress in PAUSED, and thus our live source won't be able to use the clock to produce data, making the sink never preroll and everything is stuck.
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