main/postmarketos-base-ui-gnome: add ffmpegthumbnailer to pmb_recommends

So that the file managers and choosers - Nautilus by default - show thumbnails
not only for images but also videos. Apart from being a nice-to-have, it
can help prevent privacy issues like uploading/sending wrong videos - an
increasingly realistic scenario as camera recording becomes more usable.

Note that Gnome usually uses totem-video-thumbnailer for that job,
but ffmpegthumbnailer has fewer dependencies, seems to be more stable,
is slightly faster - and pmOS already comes with ffmpeg, so there are
no additional licensing issues.

Part-of: !6300