exo: Add new owners.

Add oshima@ and dcastagna@ as owners in components/exo.

Test: 
Change-Id: I9295e888a216d52237663330abae3acf0ec60373
Reviewed-on: https://chromium-review.googlesource.com/1112297Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: David Reveman <reveman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569874}

components/exo/OWNERS

 reveman@chromium.org
+oshima@chromium.org
+dcastagna@chromium.org