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

[Vassil Vassilev via llvm-commits]

vgvassilev wrote:

> And there is one more missing puzzle piece:
> 
>     * The include of SmallVector.h (the one that fixes the problem via a parasitic include) only appears in [2222fdd#diff-3a97dc53c7250e03c83d10c7bd769776dc47bdcc170fddcd49f92f8b12f1e5f7R16](https://github.com/llvm/llvm-project/commit/2222fddfc0a2ff02036542511597839856289094#diff-3a97dc53c7250e03c83d10c7bd769776dc47bdcc170fddcd49f92f8b12f1e5f7R16) (llvm-20). So we can't fix llvm-18, because this patch doesn't apply.

This accidentally fixed it because we added `SmallVectorImpl` and had to include `SmallVector.h` and we got the include transitively.

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