[libunwind] Use relative includes to allow source-based dependencies without `-I` (PR #80443)

[Yuriy Chernyshov via cfe-commits]

georgthegreat wrote:

I understand the _this will definitely break in the future without proper testing_, but libunwind does not experience frequent changes, so this might be a working soluiton even without explicit testing.

https://github.com/llvm/llvm-project/pull/80443