net/rpc: fix build

LGTM=adg
R=golang-codereviews, adg
CC=golang-codereviews
https://golang.org/cl/151620043

src/net/rpc/client_test.go

@@ -8,6 +8,7 @@ import (
 	"errors"
 	"fmt"
 	"net"
+	"runtime"
 	"strings"
 	"testing"
 )