[llvm] [offload] Add missing build dependency (PR #149326)

[via llvm-commits]

jprotze wrote:

I just tested, whether I can use a more general dependency and tried `add_dependencies(LLVMOffload cxx-headers)` + `add_dependencies(omptarget cxx-headers)` while removing the `add_dependencies(offload-tblgen cxx-headers)` dependency.
With that change, I get the same missing header issues as before.

Empirically, I would say this dependency is both necessary and sufficient.


https://github.com/llvm/llvm-project/pull/149326