Add per-file wildcard for chromeos/BUILD.gn

So that changes to this file do not need approval from
chromeos/OWNERS

BUG=none

Review URL: https://codereview.chromium.org/882903003

Cr-Commit-Position: refs/heads/master@{#313598}

chromeos/OWNERS

 per-file chromeos_switches.cc=*
 per-file chromeos_switches.h=*
 per-file chromeos.gyp=*
+per-file BUILD.gn=*
 
 zelidrag@chromium.org
 stevenjb@chromium.org