Adding bratell@opera.com as owner of build/config/*jumbo*

Original developer and main maintainer.

Change-Id: Ia7eacd431f2d83c30ffb5231745b8fcfab8ebd21
Reviewed-on: https://chromium-review.googlesource.com/c/1297135Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#602362}

build/config/OWNERS

 dpranke@chromium.org
 scottmg@chromium.org
 
+per-file *jumbo*=bratell@opera.com
+
 per-file BUILDCONFIG.gn=dpranke@chromium.org
 per-file BUILDCONFIG.gn=set noparent