[OWNERS] Add OWNERS file for `infra/config`

Transitioning from `find-owners` to `code-owners` introduces the
expectation that there is an OWNERS file (at the root) for each
branch in a repository where OWNERS is enforced.

Duplicate the OWNERS file from `main` to `infra/config` to satisfy
this requirement here.

BUG: b/200151776
Change-Id: I5f486aefd8e6e574b4ea4e13497b074b148619ee
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/12260
Reviewed-by: Petr Hosek <phosek@google.com>