etcd-top: make build require -tags pcap

Fixes travis.

tools/etcd-top/etcd-top.go

@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// +build pcap
+
 package main
 
 import (

tools/etcd-top/stub.go

@@ -0,0 +1,23 @@
+// Copyright 2015 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build !pcap
+
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println("please build etcd-top with -tags pcap")
+}