Hide monobench target on debug builds.

Unlike nanobench this tool has no purpose when built in Debug mode.
Just don't let it happen.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2718

Change-Id: Iaa7b8c44d46024485d4f5ce3d9c3e33d865b99d7
Reviewed-on: https://skia-review.googlesource.com/2718
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
1 file changed