[llvm] [Nomination] Add ST representative to Security group (PR #93176)

[Kristof Beyls via llvm-commits]

kbeyls wrote:

If I counted correctly, we've got 9 approvals so far.
We currently have 20 people on the security team, so per https://llvm.org/docs/Security.html#choosing-new-members, we'll need another 1 or 2 approvals from existing security group members.

https://github.com/llvm/llvm-project/pull/93176