Add basic Xamarin.iOS support

csharp/Facebook.Yoga/YogaLogger.cs

@@ -14,18 +14,21 @@ namespace Facebook.Yoga
 {
     internal static class YogaLogger
     {
+#if !__IOS__
         [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
         public delegate void Func(YogaLogLevel level, string message);
-
+#endif
         private static bool _initialized;
+#if !__IOS__
         private static Func _managedLogger = null;
 
         public static Func Logger = null;
-
+#endif
         public static void Initialize()
         {
             if (!_initialized)
             {
+#if !__IOS__
                 _managedLogger = (level, message) => {
                     if (Logger != null)
                     {
@@ -38,6 +41,7 @@ namespace Facebook.Yoga
                     }
                 };
                 Native.YGInteropSetLogger(_managedLogger);
+#endif
                 _initialized = true;
             }
         }