[libc] [llvm] [libc] Add maintainers file (PR #133471)

[via llvm-commits]

PiJoules wrote:

> > @PiJoules for stdfix?
> 
> If he wants to I'm willing to add him, but he can also propose his own CL after this one has landed.

Yeah I'd love to be an owner :)

https://github.com/llvm/llvm-project/pull/133471