Add fixtures build tag to all fixtures.go files

Ensure testing dependencies from fixtures don't leak into builds.
diff --git a/rackspace/lb/v1/sessions/fixtures.go b/rackspace/lb/v1/sessions/fixtures.go
index 077ef04..e59dd6c 100644
--- a/rackspace/lb/v1/sessions/fixtures.go
+++ b/rackspace/lb/v1/sessions/fixtures.go
@@ -1,3 +1,5 @@
+// +build fixtures
+
 package sessions
 
 import (