2017/state-of-go/stdlib/sort: fix build for Go 1.7 and below

Change-Id: Ic57fac2538bba21501e9a2503a60804b9161d2cc
Reviewed-on: https://go-review.googlesource.com/37387
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

2017/state-of-go/stdlib/sort/sort_test.go

diff --git a/2017/state-of-go/stdlib/sort/sort_test.go b/2017/state-of-go/stdlib/sort/sort_test.go
index 6e58d64..1cc20fb 100644
--- a/2017/state-of-go/stdlib/sort/sort_test.go
+++ b/2017/state-of-go/stdlib/sort/sort_test.go
@@ -1,3 +1,5 @@
+// +build go1.8
+
 package main
 
 import (