chore: ignore ffi bindings for code coverage

[JKRhb]

This PR lets the code coverage tool ignore the auto-generated bindings, some of which might not get used in practice.

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: +0.24 :tada:

Comparison is base (dd2b730) 81.83% compared to head (a4af2bf) 82.07%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #52 +/- ## ========================================== + Coverage 81.83% 82.07% +0.24% ========================================== Files 10 10 Lines 666 664 -2 ========================================== Hits 545 545 + Misses 121 119 -2 ``` | [Impacted Files](https://codecov.io/gh/JKRhb/dtls2/pull/52?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jan+Romann) | Coverage Δ | | |---|---|---| | [lib/src/generated/ffi.dart](https://codecov.io/gh/JKRhb/dtls2/pull/52?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jan+Romann#diff-bGliL3NyYy9nZW5lcmF0ZWQvZmZpLmRhcnQ=) | `84.42% <ø> (+0.84%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jan+Romann). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jan+Romann)

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[JKRhb]

Hmm. Seems not to work. Closing this PR.