Fix build with FFmpeg 7

[cho-m]

Conditionals for FFmpeg 7

• pkt_duration - FFmpeg/FFmpeg@4397f9a / FFmpeg/FFmpeg@b8fef7e
• channels - FFmpeg/FFmpeg@086a804 / FFmpeg/FFmpeg@65ddc74

Versions ranges chosen based on submodule:

• https://github.com/valbok/QtAVPlayer/blob/master/src/QtAVPlayer/qavaudioconverter.cpp
• https://github.com/valbok/QtAVPlayer/blob/master/src/QtAVPlayer/qavframe.cpp

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Only verified that it builds and app runs. Not sure if there is a test suite to verify it works as expected.

FFmpeg 8 would need more work as libpostproc is removed along with other deprecated APIs.