Remove .dll from DllName for other platforms

Summary:
- Different platforms have different naming conventions
  - http://www.mono-project.com/docs/advanced/pinvoke/#library-names
- Unity iOS requires special name `__Internal`
  - https://docs.unity3d.com/Manual/PluginsForIOS.html

Reviewed By: emilsjolander

Differential Revision: D3976369

fbshipit-source-id: 4c1d7fe226c5c2a89531ee1c2ee8b47df847b3e4

csharp/Facebook.CSSLayout/Native.cs

@@ -14,7 +14,11 @@ namespace Facebook.CSSLayout
 {
     internal static class Native
     {
-        private const string DllName = "CSSLayout.dll";
+#if UNITY_IOS && !UNITY_EDITOR
+        private const string DllName = "__Internal";
+#else
+        private const string DllName = "CSSLayout";
+#endif
 
         [DllImport(DllName)]
         public static extern IntPtr CSSNodeNew();