Properly test for the lack of node measure func

Sorry Hack Gods for putting this back to work :)

YogaKit/Source/YGLayout.m

@@ -180,7 +180,7 @@ static YGConfigRef globalConfig;
   // the measure function. Since we already know that this is a leaf,
   // this *should* be fine. Forgive me Hack Gods.
   const YGNodeRef node = self.node;
-  if (YGNodeHasMeasureFunc(node)) {
+  if (!YGNodeHasMeasureFunc(node)) {
     YGNodeSetMeasureFunc(node, YGMeasureView);
   }