avfvideosrc: Explicitly request video permission on macOS 10.14+

So far gstreamer wouldn't display any webcam on Mojave because permissions now have to be granted explicitly.

I have tested the video permission on my Macbook, but couldn't test audio permissions so far.

I have tested the patch by running gst-launch-1.0 avfvideosrc ! videoconvert ! autovideosink.

The ask permission popup can be viewed in this screenshot and the video opening can be viewed in this one

It's my first Merge Request on this project, so I'm not sure the patch respects the guidelines :/ . Please let me know if there's anything I can do to improve the pull request, and thank you for your time.

Pinging (and thanking!) @slomo who has supported me and provided me guidelines on how to setup the environment and start working on the patch :)