Fix bugs introduced with YogaKit improvements

Summary:
I'm trying to fix some bugs I introduced in my latest PR, but while writing the Unit Tests for them, I saw a really weird behaviour. The following exact piece of code WORKS inside a Yoga C++ unit test, but fails from a Objective-C unit test. I had me completely confused and blocked me in my progression. Any ideas?

```
TEST(YogaTest, stupid_test) {
  const YGNodeRef node = YGNodeNew();
  YGNodeStyleSetPosition(node, YGEdgeLeft, 1);

  ASSERT_FLOAT_EQ(1, YGNodeStyleGetPosition(node, YGEdgeLeft).value);
  ASSERT_EQ(YGUnitPixel, YGNodeStyleGetPosition(node, YGEdgeLeft).unit);

  YGNodeFree(node);
}

```

```
- (void)testPositionalPropertiesWork
{
  YGNodeRef node = YGNodeNew();
  YGNodeStyleSetPosition(node, YGEdgeLeft, 1);

  XCTAssertEqual(1, YGNodeStyleGetPosition(node, YGEdgeLeft).value);
  XCTAssertEqual(YGUnitPixel, YGNodeStyleGetPosition(node, YGEdgeLeft).unit);

  YGNodeFree(node);
}
```
Closes https://github.com/facebook/yoga/pull/328

Reviewed By: dshahidehpour

Differential Revision: D4421504

Pulled By: emilsjolander

fbshipit-source-id: f59379edf70aee87a77cd1ad2986313cdfe71b94

YogaKit/YGLayout+Private.h

@@ -8,8 +8,11 @@
  */
 
 #import "YGLayout.h"
+#import <yoga/Yoga.h>
 
-@interface YGLayout (Private)
+@interface YGLayout ()
+
+@property (nonatomic, assign, readonly) YGNodeRef node;
 
 - (instancetype)initWithView:(UIView *)view;