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

[via llvm-commits]

jprotze wrote:

> This change looks fine, but is there a reason it only affects `offload-tblgen`? Could other offload components also run into this issue? I'm wondering if it's possible to capture the dependency at the level of the entire runtime when multiple are being built (e.g. `openmp` and `offload` might depend on `libcxx`)

I was also a bit surprised, that adding this specific dependency fixes the build reproducibly (I did multiple clean builds with 100 processes). I think, that simply all other liboffload code depends on `offload-tblgen` ?

`libomp` is not affected, because we carefully avoid dependencies to a C++ runtime library.

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