[ast] Add leszeks@ to src/ast owners

Change-Id: I64b2ae64dc668a937be0bf2950199b4f3d543f50
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2020778
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66002}

src/ast/OWNERS

 adamk@chromium.org
 bmeurer@chromium.org
 gsathya@chromium.org
+leszeks@chromium.org
 littledan@chromium.org
 marja@chromium.org
 neis@chromium.org