Update libc to 0.2.17
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Update `libc` to pick up rust-lang/libc#424.
(This is a fix for servo/servo#13154)
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix#13154.
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This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/13857)
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99ad3678fa
