Add oomichi to reviwers of MetalLB addon (#6393)

I'd like to review PRs related to metallb addon as possible to make
it better, and it would be easy to track related PRs if becoming the
reviewer.

roles/kubernetes-apps/metallb/OWNERS

+# See the OWNERS docs at https://go.k8s.io/owners
+
+approvers:
+reviewers:
+  - oomichi