Since 6a23ae168f, 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
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1854>