This will allow it to depend on whether plugin.Open is a symbol to be
linked in.

Change-Id: Ie9aa4216f2510fe8b10bc4665c8b19622b7122ea
Reviewed-on: https://go-review.googlesource.com/27819Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>