Add syg@ as ast/ OWNER

Change-Id: Ie74e9bb523463a4c9a0f23a1788246b376e08b14
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3543169Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79576}

src/ast/OWNERS

 leszeks@chromium.org
 marja@chromium.org
+syg@chromium.org
 verwaest@chromium.org