[llvm] [Support] System include SipHash.h (PR #149499)

[Peter Collingbourne via llvm-commits]

pcc wrote:

Would it also work to remove the SYSTEM directive? That seems like the right thing to do because this is isn't a system header. I don't think I added SYSTEM deliberately, it was copy-pasted from the previous target_include_directories for Z3.

https://github.com/llvm/llvm-project/pull/149499