cmake: fix ffmpeg 5.1 detection

[cho-m]

Description

Fix CMake detection of FFmpeg 5.1, which refactored location of libavcodec major version in FFmpeg/FFmpeg@f2da2e1. Seen in Homebrew Homebrew/homebrew-core#107244

Only fixes detection and I confirmed successful build. Not sure if any breaking changes in API, but at least still compiles with newer FFmpeg.

Original cmake output for master branch with FFmpeg 5.1:

❯ PKG_CONFIG_PATH=/usr/local/opt/ffmpeg/lib/pkgconfig cmake -S. -Bmaster 2>/dev/null | grep -i ffmpeg
-- FFmpeg 1.0 is outside the required range 3.0...
-- FFmpeg library not found
--     Try setting FFmpeg_ROOT ?
-- FFmpeg not found: ffmpeg plugin will not be built

And same with changes:

❯ PKG_CONFIG_PATH=/usr/local/opt/ffmpeg/lib/pkgconfig cmake -S. -Bfix 2>/dev/null | grep -i ffmpeg
-- Found FFmpeg 5.1

And confirming FFmpeg 4 is still detected:

❯ PKG_CONFIG_PATH=/usr/local/opt/ffmpeg@4/lib/pkgconfig cmake -S. -Bfix2 2>/dev/null | grep -i ffmpeg
-- Found FFmpeg 4.4

Tests

Checklist:

• I have read the contribution guidelines.
• If this is more extensive than a small change to existing code, I
  have previously submitted a Contributor License Agreement
  (individual, and if there is any way my
  employers might think my programming belongs to them, then also
  corporate).
• I have updated the documentation, if applicable.
• I have ensured that the change is tested somewhere in the testsuite
  (adding new test cases if necessary).
• My code follows the prevailing code style of this project.

[lgritz]

LGTM, thanks for the fix!