Add fixtures build tag to all fixtures.go files

Ensure testing dependencies from fixtures don't leak into builds.
diff --git a/rackspace/db/v1/instances/fixtures.go b/rackspace/db/v1/instances/fixtures.go
index c5ff37a..92d7bed 100644
--- a/rackspace/db/v1/instances/fixtures.go
+++ b/rackspace/db/v1/instances/fixtures.go
@@ -1,3 +1,5 @@
+// +build fixtures
+
 package instances
 
 import (