Mark enum with [Flags] where they are used as flags

Summary:
Marks enum with ```[Flags]``` attribute, where they are used as flags.
Closes https://github.com/facebook/yoga/pull/515

Reviewed By: emilsjolander

Differential Revision: D4921807

Pulled By: splhack

fbshipit-source-id: f056189b76ee6e6c042417a7998bfd20a610c27f

csharp/Facebook.Yoga/YogaPrintOptions.cs

@@ -9,6 +9,7 @@
 
 namespace Facebook.Yoga
 {
+    [System.Flags]
     public enum YogaPrintOptions
     {
         Layout = 1,