libtas: Fix movie encode failing to capture audio

libTAS requires libswresample to capture & encode audio of an application. Otherwise, the end result would be a silent recording.

pkgs/by-name/li/libtas/package.nix

···
       --suffix LD_LIBRARY_PATH : ${
         lib.makeLibraryPath [
           xorg.libXi
+          ffmpeg.lib
         ]
       } \
       --set-default LIBTAS_SO_PATH $out/lib/libtas.so