build: convert to go:build directives (#2664)

context_appengine.go

@@ -1,9 +1,10 @@
-// +build appengine
-
 // Copyright 2017 Manu Martinez-Almeida.  All rights reserved.
 // Use of this source code is governed by a MIT style
 // license that can be found in the LICENSE file.
 
+//go:build appengine
+// +build appengine
+
 package gin
 
 func init() {