[Code health] Enable gn check for //sandbox/linux

Bug: 1159029
Change-Id: I8456458f9abc638d741a1192f184b795b544987f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2666706


Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Erik Staab <estaab@chromium.org>
Auto-Submit: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Erik Staab <estaab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#851453}