Add sync team as owner of components/sync_preferences

This component contains sync-specific logic and plumbing so let's follow
the setup in other components/sync_* components and list the core sync
team as owners.

Bug: None
Change-Id: I509a41e3b4738967188680771ae8c535c02ca7ef
Reviewed-on: https://chromium-review.googlesource.com/c/1343261Reviewed-by: Dominic Battré <battre@chromium.org>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609658}

components/sync_preferences/OWNERS

+file://components/sync/OWNERS
 battre@chromium.org
 gab@chromium.org