[compiler-rt] [compiler-rt][XRay] Make `xray_interface.h` C compliant (PR #140068)

[Sebastian Kreutzer via llvm-commits]

https://github.com/sebastiankreutzer commented:

Thanks for submitting this PR. I think this is a very sensible change, and will make it easier for third-party C tools to use XRay going forward. 

https://github.com/llvm/llvm-project/pull/140068