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/serviceassets/fixtures.go b/openstack/cdn/v1/serviceassets/fixtures.go
index 5c6b5d0..c4739ae 100644
--- a/openstack/cdn/v1/serviceassets/fixtures.go
+++ b/openstack/cdn/v1/serviceassets/fixtures.go
@@ -1,3 +1,5 @@
+// +build fixtures
+
 package serviceassets
 
 import (