[llvm] [github][Security] Add the llvm-security-group team as code owner for Security.rst (PR #106255)

[Tulio Magno Quites Machado Filho via llvm-commits]

tuliom wrote:

> I don't think the group is _publicly_ visible -- in a private window, I'm prompted to log in first.

The same thing happens with other teams, e.g. reviewers-libcxx .
I wonder if @kbeyls can see the settings of the group.

https://github.com/llvm/llvm-project/pull/106255