google: fix the build when appengine isn't present

See https://build.golang.org/log/c3e046245c4eafbb7b2571ef9ac144b0d29ba2b5

Updates golang/go#11811

Change-Id: I16d2ac26dcda123e1bd8c456e490f6ca45111d24
Reviewed-on: https://go-review.googlesource.com/24946
Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: Chris Broadfoot <cbro@golang.org>
diff --git a/google/example_test.go b/google/example_test.go
index 9745be1..92bc3b4 100644
--- a/google/example_test.go
+++ b/google/example_test.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build appenginevm !appengine
+// +build appenginevm appengine
 
 package google_test