Move XM bindings to top level csharp folder

csharp/Mac/ApiDefinition.cs

@@ -0,0 +1,14 @@
+using System;
+
+using AppKit;
+using Foundation;
+using ObjCRuntime;
+using CoreGraphics;
+
+namespace Facebook.Yoga.Mac
+{
+	// Xamarin.Mac binding projects allow you to include native libraries inside C# DLLs for easy consumption
+	// later. However, the binding project build files currently assume you are binding some objective-c API
+	// and that you need an ApiDefinition.cs for that. yoga is all C APIs, so just include this "blank" file so
+	// the dylib gets packaged
+}