Add workaround from QTBUG-129651 for iOS FFMPEG linking

[ann0see]

Short description of changes

CHANGELOG: SKIP
Context: Fixes an issue?

Allows building (but not running) Jamulus on iOS with Qt > 6.7.3 where the ffmpeg backend was introduced.
Does this change need documentation? What needs to be documented and how?

Status of this Pull Request

Ready for testing/review (I think it's worth just merging this as iOS Qt6 is not stable anyway). We shouldn't move up for iOS yet.

What is missing until this pull request can be merged?

Checklist

• I've verified that this Pull Request follows the general code principles
• I tested my code and it does what I want (as in: It compiles now but still produces a binary which starts with a black screen)
• My code follows the style guide
• I waited some time after this Pull Request was opened and all GitHub checks completed without errors.
• I've filled all the content above