[llvm] [Runtimes][libc] Include kernel headers when using RUNIMES_USE_LIBC (PR #165114)

[Michael Jones via llvm-commits]

https://github.com/michaelrj-google approved this pull request.

LGTM, though you should probably double check how the cmake variable definition ordering works on this. LIBC_KERNEL_HEADERS is intended to be passed by the user but it also has a default value that might not get picked up on a fresh build

https://github.com/llvm/llvm-project/pull/165114