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>

src/cmd/go/internal/load/pkg.go

@@ -1343,6 +1343,7 @@ func (p *Package) load(stk *ImportStack, bp *build.Package, err error) {
 
 	// SWIG adds imports of some standard packages.
 	if p.UsesSwig() {
+		addImport("unsafe", true)
 		if cfg.BuildContext.Compiler != "gccgo" {
 			addImport("runtime/cgo", true)
 		}