Fix building with ffmpeg 4.3 and newer

[swiftgeek]

Fixes building with newer ffmpeg.

/tmp/DivvyDroid/src/device/videothread.cpp: In member function ‘int VideoThread::h264VideoStreamIndex()’:
/tmp/DivvyDroid/src/device/videothread.cpp:170:52: error: invalid conversion from ‘const AVCodec*’ to ‘AVCodec*’ [-fpermissive]
  170 |                 AVCodec *dec = avcodec_find_decoder(m_avStream->codecpar->codec_id);
      |                                ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                                                    |
      |                                                    const AVCodec*

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Similar issue resolved in some other project:

• dmlc/decord@b67c526
• Error during make: invalid conversion from 'const AVCodec*' to 'AVCodec*' dmlc/decord#155

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Change tested with recent archlinux, ffmpeg 5.0 and android 10/11

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Thx for this project, seems to be more reliable than scrcpy thanks to many fallbacks

[maxrd2]

thank you... could you please use const AVCodec * instead of auto