the build tags in comments removed by gen command

[bigwhite]

If the source file has comments , after handling by gen command, the comments will be removed. if the source file has build tags, the build tags will be also removed. this could cause compile error when build.

origin source file:

// Copyright (c) 2019 Andy Pan
   Copyright (c) 2018 Joshua J Baker
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.

// +build linux freebsd dragonfly darwin

package gnet

import (
    "runtime"
    "sync"
    "sync/atomic"
    "time"

    "github.com/panjf2000/gnet/errors"
    "github.com/panjf2000/gnet/internal/logging"
    "github.com/panjf2000/gnet/internal/netpoll"
)

... 

after gen, it will look like below：

package gnet
  
import (
    "runtime"
    "sync"
    "sync/atomic"
    "time"

    "github.com/bigwhite/functrace"
    "github.com/panjf2000/gnet/errors"
    "github.com/panjf2000/gnet/internal/logging"
    "github.com/panjf2000/gnet/internal/netpoll"
)