kde4: Use ffmpeg_2 to fix ffmpegthumbs build

pkgs/top-level/all-packages.nix

···
           pythonBindings = true;
         };
         ruby = ruby_2_2; # see https://github.com/NixOS/nixpkgs/pull/12610#issuecomment-188666473
+        ffmpeg = ffmpeg_2; # ffmpegthumb doesn't build otherwise
       }
       ../desktops/kde-4.14;