Files app: Add Josh to OWNERS

Josh has become committer congrats!

Change-Id: I0b39ecd0b9b1b755aac8fa4a92d8ac30f28c1b2f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2536215Reviewed-by: Josh Simmons <simmonsjosh@google.com>
Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827163}

ui/file_manager/OWNERS

@@ -9,5 +9,6 @@ joelhockey@chromium.org
 lucmult@chromium.org
 noel@chromium.org
 sammc@chromium.org
+simmonsjosh@google.com
 slangley@chromium.org
 # COMPONENT: Platform>Apps>FileManager