Fix deprecated FFmpeg functions

[Podre-Henrique]

Warnings when building with FFmpeg support:

whisper.cpp/examples/ffmpeg-transcode.cpp:282:23: warning: ‘void av_init_packet(AVPacket*)’ is deprecated [-Wdeprecated-declarations]
  282 |         av_init_packet(&packet);
      |         ~~~~~~~~~~~~~~^~~~~~~~~
In file included from /usr/include/libavcodec/avcodec.h:44,
                 from whisper.cpp/examples/ffmpeg-transcode.cpp:28:
/usr/include/libavcodec/packet.h:670:6: note: declared here
  670 | void av_init_packet(AVPacket *pkt);
      |      ^~~~~~~~~~~~~~
whisper.cpp/examples/ffmpeg-transcode.cpp:307:22: warning: ‘int avcodec_close(AVCodecContext*)’ is deprecated [-Wdeprecated-declarations]
  307 |         avcodec_close(codec);
      |         ~~~~~~~~~~~~~^~~~~~~
/usr/include/libavcodec/avcodec.h:2398:5: note: declared here
 2398 | int avcodec_close(AVCodecContext *avctx);

Replace av_init_packet with av_packet_alloc
Replace avcodec_close with avcodec_free_context
Also add packet cleanup with av_packet_free