dep: Update bindgen to 0.69

[chrysn]

This is a regular "let's not leave old dependencies around" update with no real changes.

Going to 0.70 would be great, but so far introduces breakage around the size of max_align_t (might be related to the new --override-abi switch).

[chrysn]

While 0.69 is recent enough to use wrap_static_fns, I hope the next jab at this goes right to 0.71

[chrysn]

Rebased onto #59; this should have been in long ago, but the first rebase onto latest main showed the problem that is now solved by #59.