I'm seeing this too, but on 6.6.3 (6.6.2 is fine).
Bisected it down to commit 2e8b4e0992e16 ("gcc-plugins: randstruct:
Only warn about true flexible arrays"). Reverting that commit on top
of v6.6.3 makes it go away.
I do wonder if content such as that (which *looks* like it's purely
preparing for future changes) is appropriate for the stable trees.
Cheers,
-- Dan