Fix FFmpeg build buster on Windows.

[acolwell]

Thanks for submitting a pull request! Please provide enough information so that others can review your pull request. Additionally, make sure you've done all of these things:

• [x] I've followed the contributing guidelines to the best of my understanding
• [x] I've read and understood the contributing guidelines
• [x] I've formatted my code according to Natron's code style
• [x] I've searched the pull requests tracker to ensure that this PR is not a duplicate

PR Description

What type of PR is this? (Check one of the boxes below)

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Improvement (non-breaking change which does not add functionality nor fixes a bug but improves Natron in some way)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] My change requires a change to the documentation
  • [ ] I have updated the documentation accordingly

What does this pull request do?

MinGW started using a newer version of binutils that breaks the ffmpeg build. This change adds the patch that MinGW uses to address this issue in its own FFmpeg package.

Have you tested your changes (if applicable)? If so, how?

Yes. I built the ffmpeg package locally and also verified that the "build pacman repo" GitHub Action builds again (https://github.com/acolwell/Natron/actions/runs/5779385803). This action should also build when this pull request is submitted.

[devernay]

@rodlie if you don't have bandwidth for these reviews, maybe @acolwell should be named "Windows buildmaster", with the authorization to push changes to the buildsystem without code review (as I do for macOS)?