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

[Callum Fare via llvm-commits]

callumfare 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`)

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