webrtcsink: fix custom_signaller hanging
Since 6a23ae16, the chain function of webrtcsink adds a custom meta on input buffers.
That custom meta was registered only by the class_init of the subclasses of BaseWebRTCSink, but the custom signaller example uses BaseWebRTCSink::with_signaller() directly.
Fix by registering the meta in BaseWebRTCSink::class_init()
Fixes: #610 (closed)