Hide init of YGLayout

Summary:
Make default init unavailable, as it will not initialise `YGNode` which is required for the setters
 and getters of YGLayout's properties to work properly. In setters and getters we call the corresponding `YGNode`'s setters and getters. Thus the following code will crash

```
let layout = YGLayout()
layout.flexDirection = .row
```

Reviewed By: emilsjolander

Differential Revision: D6597796

fbshipit-source-id: 653d275d7314bfdd7a1106884fa3fbc0a13e1be7

YogaKit/Source/YGLayout.h

@@ -29,7 +29,15 @@ typedef NS_OPTIONS(NSInteger, YGDimensionFlexibility) {
 @interface YGLayout : NSObject
 
 /**
-  The property that decides if we should include this view when calculating layout. Defaults to YES.
+ Make default init unavailable, as it will not initialise YGNode which is
+ required for the setters and getters of YGLayout's properties to work properly.
+*/
+- (instancetype)init
+    __attribute__((unavailable("you are not meant to initialise YGLayout")));
+
+/**
+  The property that decides if we should include this view when calculating
+  layout. Defaults to YES.
  */
 @property (nonatomic, readwrite, assign, setter=setIncludedInLayout:) BOOL isIncludedInLayout;