ffmpeg: Enable vid-stab by default

[Vidstab][1] is an useful ffmpeg video stabilization filter.

Both [Debian][2] and [Archlinux][3] enable it on default ffmpeg build,
we should also enable it on default ffmpeg in nixpkgs.

On the `master` branch, closure size for ffmpeg-headless went up 182.9KiB.
```
$ nix store diff-closures nixpkgs#ffmpeg-headless^bin .#ffmpeg-headless^bin
ffmpeg-headless: +18.1 KiB
vid.stab-unstable: ∅ → 2022-05-30, +164.8 KiB

$ nvd diff $(nix build nixpkgs#ffmpeg-headless^bin --print-out-paths --no-link) $(nix build .#ffmpeg-headless^bin --print-out-paths --no-link)
<<< /nix/store/kwihalx7ryh51ghcp8f1hhy8skbdh8w9-ffmpeg-headless-6.1.2-bin
>>> /nix/store/ps62y85p9jgjbf2x9s97199mpqnd0ggz-ffmpeg-headless-6.1.2-bin
Added packages:
[A.]  #1  vid.stab-unstable  2022-05-30
Closure size: 78 -> 79 (4 paths added, 3 paths removed, delta +1, disk usage +182.9KiB).
```

[1]: http://public.hronopik.de/vid.stab/
[2]: https://salsa.debian.org/multimedia-team/ffmpeg/-/blob/debian/7%256.1.1-5/debian/control#L158
[3]: https://gitlab.archlinux.org/archlinux/packaging/packages/ffmpeg/-/blob/2-6.1.1-7/PKGBUILD?ref_type=tags#L73

pkgs/development/libraries/ffmpeg/generic.nix

@@ -119,7 +119,7 @@
 , withVaapi ? withHeadlessDeps && (with stdenv; isLinux || isFreeBSD) # Vaapi hardware acceleration
 , withVdpau ? withSmallDeps && !stdenv.hostPlatform.isMinGW # Vdpau hardware acceleration
 , withVideoToolbox ? withHeadlessDeps && stdenv.isDarwin # Apple VideoToolbox
-, withVidStab ? withFullDeps && withGPL # Video stabilization
+, withVidStab ? withHeadlessDeps && withGPL # Video stabilization
 , withVmaf ? withFullDeps && !stdenv.isAarch64 && lib.versionAtLeast version "5" # Netflix's VMAF (Video Multi-Method Assessment Fusion)
 , withVoAmrwbenc ? withFullDeps && withVersion3 # AMR-WB encoder
 , withVorbis ? withHeadlessDeps # Vorbis de/encoding, native encoder exists