Errors when building Qt with FFMpeg/vaapi on Ubuntu 20.04

Since we moved from Qt 6.5.2 to 6.5.3 Qt no longer builds with FFmpeg/vaapi on Ubuntu 20.04. Ubuntu 22.04 is OK. I'm assuming this is because of the difference in versions of libva on these distros (v2.7 and v2.14 respectively).

There are a bunch of errors like this:

In file included from /home/martinb/workspace/qt6_local_build/src/qt6/qtmultimedia/src/plugins/multimedia/ffmpeg/qffmpegvaapisymbols.cpp:6:
/home/martinb/workspace/qt6_local_build/src/qt6/qtmultimedia/src/plugins/multimedia/ffmpeg/qffmpegvaapisymbols.cpp:82:13: error: ‘vaSyncBuffer’ was not declared in this scope; did you mean ‘vaMapBuffer’?
82 | DEFINE_FUNC(vaSyncBuffer, 3, VA_STATUS_ERROR_OPERATION_FAILED);
   |             ^~~~~~~~~~~~

We don't necessarily need vaapi so for us the workaround is to disable vaapi ie. --disable-vaapi when building FFmpeg and -no-feature-vaapi when building Qt.