Fixes for pkg-config Requires

Hi there,

While building gstreamer + gst-plugins-base + -good + -bad from source to build gst-rtsp-server, I had a couple of build failures, probably limited to the "uninstalled" build scenarios.

Attaching snippets with relevant part of build log. The proposed merge proposal fixes both issues.

Thanks!

- Loïc

gst-transcoder-build-error.txt webrtc-build-error.txt