[meta-sig] Add Thierry Carrez to core reviewer

I would like to propose adding Thierry to core reviewer for governance-sigs I don't think anyone will doubt his knowledge of maintaining governance-sigs repo. He will be able to do core review on all patches. To create SIG patch, it will require both UC and TC to give their blessing. That means besides Amy's +2, we only need Thierry or mine +2 to approve the workflow.

Let me know what you think if you have other thoughts, otherwise, I will add him in core think week.