frei0r-plugins should be a dependency

The default ffmpeg installation is configured with --enable-frei0r, but the frei0r-plugins package isn't installed with it as a dependency like it should. This means that the frei0r video filters can't be used until frei0r-plugins is manually installed, despite ffmpeg saying it is configure with it enabled.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: libavfilter7 7:4.2.4-1ubuntu0.1
ProcVersionSignature: Ubuntu 5.4.0-66.74-generic 5.4.86
Uname: Linux 5.4.0-66-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.16
Architecture: amd64
CasperMD5CheckResult: pass
Date: Thu Mar 11 06:32:50 2021
InstallationDate: Installed on 2021-03-02 (8 days ago)
InstallationMedia: Ubuntu-Server 20.04.2 LTS "Focal Fossa" - Release amd64 (20210201.2)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: ffmpeg
UpgradeStatus: No upgrade log present (probably fresh install)