xsrftoken: add package import comment

Change-Id: I8548d0e7a9400fd7a02c761b0b339749b69fbe31
Reviewed-on: https://go-review.googlesource.com/17665
Reviewed-by: David Symonds <dsymonds@golang.org>

xsrftoken/xsrf.go

@@ -3,7 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // Package xsrftoken provides methods for generating and validating secure XSRF tokens.
-package xsrftoken
+package xsrftoken // import "golang.org/x/net/xsrftoken"
 
 import (
 	"crypto/hmac"