`ptrauth.h` is missing

Ran into this while building [libffi](https://github.com/libffi/libffi). It has support for pointer authentication on Arm64, but it needs [`ptrauth.h`](https://opensource.apple.com/source/xnu/xnu-4903.241.1/EXTERNAL_HEADERS/ptrauth.h.auto.html) to build successfully with it.