gstreamer1.0-libav: fix errors with ffmpeg 5.x

Backport of patch already present upstream to fix issues with invalid
characters for GLIB when combining gstreamer1.0-libav with ffmpeg 5.x.

Remove when gstreamer1.0-libav is upgraded to 1.21.1 or above

Signed-off-by: Claus Stovgaard <claus.stovgaard@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>