qt6.qtmultimedia: fix build on aarch64-darwin

[wegank]

Description of changes

https://hydra.nixos.org/job/nixpkgs/staging-next/qt6.qtmultimedia.aarch64-darwin

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.05 Release Notes (or backporting 22.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[wegank]

@ofborg build qt6.qtmultimedia qt6.qtmultimedia.passthru.tests

[NickCao]

According to the qt 6.5 release note: https://www.qt.io/blog/qt-6.5-lts-released#qt-multimedia, the default backend for qtmultimedia is now ffmpeg instead of gstreamer. Shall we migrate to ffmpeg too? Supposedly it would also fix the build on darwin, as this pr originally intends.

[wegank]

Shall we migrate to ffmpeg too?

Honestly, I have no idea if this will break any packages. Will test when Hydra finishes building qt6.qtmultimedia on aarch64-linux.