Add owners file to gcmole folder

Narrow down the list of owners to choose from in the gcmole auto-update CLs.

Bug: v8:11169
Change-Id: I9f553124c43dc9fc094a8f4a6e2bcc832fe27636
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3236993
Commit-Queue: Liviu Rau <liviurau@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77509}

tools/gcmole/OWNERS

+clemensb@chromium.org
+leszeks@chromium.org
+machenbach@chromium.org
+mslekova@chromium.org
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