Update source to match output from gofmt 1.17

Author: garyburd

client_clone.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build go1.8
 // +build go1.8
 
 package websocket

client_clone_legacy.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build !go1.8
 // +build !go1.8
 
 package websocket

conn_write.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build go1.8
 // +build go1.8
 
 package websocket

conn_write_legacy.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build !go1.8
 // +build !go1.8
 
 package websocket

examples/echo/client.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build ignore
 // +build ignore
 
 package main

examples/echo/server.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build ignore
 // +build ignore
 
 package main

mask.go

@@ -2,6 +2,7 @@
 // this source code is governed by a BSD-style license that can be found in the
 // LICENSE file.
 
+//go:build !appengine
 // +build !appengine
 
 package websocket

mask_safe.go

@@ -2,6 +2,7 @@
 // this source code is governed by a BSD-style license that can be found in the
 // LICENSE file.
 
+//go:build appengine
 // +build appengine
 
 package websocket

trace.go

@@ -1,3 +1,4 @@
+//go:build go1.8
 // +build go1.8
 
 package websocket

trace_17.go

@@ -1,3 +1,4 @@
+//go:build !go1.8
 // +build !go1.8
 
 package websocket