runtime: add blank assignment to defeat "declared but not used" error from go/types

gc should ideally consider this an error too; see golang/go#8560.

Change-Id: Ieee71c4ecaff493d7f83e15ba8c8a04ee90a4cf1
Reviewed-on: https://go-review.googlesource.com/10757Reviewed-by: Robert Griesemer <gri@golang.org>

src/runtime/stack_test.go

@@ -329,6 +329,7 @@ func TestPanicFar(t *testing.T) {
 		time.Sleep(time.Millisecond)
 		panic(1)
 	})
+	_ = xtree
 }
 
 type xtreeNode struct {