wlroots_0_12: fixup build with ffmpeg-4.4
wlroots_0_12: fixup build with ffmpeg-4.4

-Werror=deprecated-declarations is nice upstream but not in distro.
https://nix-cache.s3.amazonaws.com/log/cfmk2qkvck88qc002bvxzgsb2zc5b1h2-wlroots-0.12.0.drv

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs