Add fixtures build tag to all fixtures.go files

Ensure testing dependencies from fixtures don't leak into builds.
diff --git a/rackspace/identity/v2/roles/fixtures.go b/rackspace/identity/v2/roles/fixtures.go
index 5f22d0f..e194e31 100644
--- a/rackspace/identity/v2/roles/fixtures.go
+++ b/rackspace/identity/v2/roles/fixtures.go
@@ -1,3 +1,5 @@
+// +build fixtures
+
 package roles
 
 import (