[iOS] Add stkhapugin as owner of i/c/b/ui/incognito_reauth.

Reason: I wrote all of this code.

Bug: none
Change-Id: I91d5df3b88c8f1845bd89f3b1fbbeb6c4d8d2bb3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2595048Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#838151}

ios/chrome/browser/ui/incognito_reauth/OWNERS

+stkhapugin@chromium.org