Add mukai@ as input related OWNERS in exo

Bug: None
Change-Id: Ida7a9e313069a8a7dc5844ab1e5f65a1c4a466d0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2325230Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#792489}

components/exo/OWNERS

 oshima@chromium.org
 dcastagna@chromium.org
+
+# For Input (keyboard, mouse, touch)
+mukai@chromium.org
+
 # COMPONENT: Internals>Exosphere