rtpfunnel: Stop using G_DECLARE_FINAL_TYPE

Fixes #516
2 jobs for rtpfunnel-glib-2.40 in 2 minutes and 29 seconds (queued for 3 seconds)
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  Manifest
passed #43176
manifest

00:01:06

 
  Build
passed #43177
build fedora x86_64

00:01:23