cmd/go: packages that use SWIG depend on "unsafe"

Fixes #28834

Change-Id: I95d6874e62d36974415f43843881a4ae85b3c7ce
Reviewed-on: https://go-review.googlesource.com/c/149964


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>