Adding luum to chromeos/printing OWNERS

Bug: none
Test: na
Change-Id: I09a09bcc6bc63be671a19b375c0c5afc4ba0f46b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1717744Reviewed-by: Sean Kau <skau@chromium.org>
Commit-Queue: Luum Habtemariam <luum@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681486}

chromeos/printing/OWNERS

 skau@chromium.org
 thestig@chromium.org
 zentaro@chromium.org
+luum@chromium.org
 
 #TEAM: cros-printing-dev@chromium.org
 #COMPONENT: Internals>Printing>CUPS