pipewire: loop: Use `dyn Fn`s instead of monomorphization to store loop source callbacks.
The functions were already stored in a Box
before, so the extra indirection isn't too bad.
As the type of closures can't be named, this made working with sources before unnecessarily hard.
See for example https://gitlab.freedesktop.org/ryuukyu/helvum/-/issues/36#note_1212404