Add donnd@ to partnercustomizations OWNERS

Change-Id: I6affae350a371ebb6161b77826658b8e2c1b72cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4754914


Reviewed-by: Donn Denman <donnd@chromium.org>
Commit-Queue: Wenyu Fu <wenyufu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1180464}