libsForQt5.ffmpegthumbs: ffmpeg_3 -> ffmpeg_4 (#117859)

ffmpegthumbs is no longer compatibile with ffmpeg post
v20.11.80/v21.03.80 due to dropping a call to av_register_all(). Bump
ffmpeg dependency to match new required version.

pkgs/applications/kde/ffmpegthumbs.nix

···
 {
   mkDerivation, lib,
   extra-cmake-modules,
-  ffmpeg_3, kio, taglib
+  ffmpeg_4, kio, taglib
 }:
 
 mkDerivation {
···
     maintainers = [ lib.maintainers.ttuegel ];
   };
   nativeBuildInputs = [ extra-cmake-modules ];
-  buildInputs = [ ffmpeg_3 kio taglib ];
+  buildInputs = [ ffmpeg_4 kio taglib ];
 }