[owners] Add ishell to zone/OWNERS

Change-Id: I31e16fcfd6641d88b3b6d3195dc30464833d0821
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2231345
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68195}

src/zone/OWNERS

 clemensb@chromium.org
+ishell@chromium.org
 sigurds@chromium.org
 verwaest@chromium.org