Fix FindAVDEVICE.cmake in case without pkg-config installed with ffmpeg >= 5.1

[traversaro]

🦟 Bug fix

Summary

ffmpeg 5.1 changed the structure of version headers (see FFmpeg/FFmpeg@884c597), this PR fixes FindAVDEVICE.cmake to work correctly (i.e. finding the version number) even in the case in which no pkg-config is installed in the system.

The GitHub online editor also added a newline at the end of the file, let me know if it is ok to leave it, otherwise I can look how to remove it. I tried but apparently it is not possible anymore to commit a change via GitHub online editor without having automatically added a newline at the end of file.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[j-rivero]

ffmpeg 5.1 changed the structure of version headers (see FFmpeg/FFmpeg@884c597),

Ummm it was 51 lines trivial text file used by many, I'm going to think that there was a good reason to do it :)

The GitHub online editor also added a newline at the end of the file, let me know if it is ok to leave it, otherwise I can look how to remove it. I tried but apparently it is not possible anymore to commit a change via GitHub online editor without having automatically added a newline at the end of file.

I tried to be clever than the editor and failed miserably. They probably are now trying to be serious with the POSIX standard on text files. I'm not here to go again the standards and to waste your time so let's consider it fine unless someone want's to argue the opposite.