[llvm] Add missing include to X86MCTargetDesc.h (PR #123320)

[Vassil Vassilev via llvm-commits]

vgvassilev wrote:

Generally self contained header files are a good practice. While we don't really enforce that in the LLVM development policy we implicitly required self contained header files for clang modules. It is great if gcc-15 became more strict in that regard. I think we should move forward with this trivial patch if it solves something for downstream clients.

https://github.com/llvm/llvm-project/pull/123320