Add fixtures build tag to all fixtures.go files

Ensure testing dependencies from fixtures don't leak into builds.
diff --git a/openstack/cdn/v1/services/fixtures.go b/openstack/cdn/v1/services/fixtures.go
index d9bc9f2..3d53900 100644
--- a/openstack/cdn/v1/services/fixtures.go
+++ b/openstack/cdn/v1/services/fixtures.go
@@ -1,3 +1,5 @@
+// +build fixtures
+
 package services
 
 import (