io/i2c: fix the broken build

Change-Id: Ie1a5a21d947f25dde260b7264260295662476406
Reviewed-on: https://go-review.googlesource.com/22643
Reviewed-by: Jaana Burcu Dogan <jbd@google.com>
diff --git a/io/i2c/example_test.go b/io/i2c/example_test.go
index 535608c..114de32 100644
--- a/io/i2c/example_test.go
+++ b/io/i2c/example_test.go
@@ -19,4 +19,6 @@
 	if err != nil {
 		panic(err)
 	}
+
+	_ = d
 }