Remove compiler warning (#23588)

Summary:
Fixed compiler warning, after this, seems we have no warning for framework in debug mode.
<img width="1280" alt="image" src="https://user-images.githubusercontent.com/5061845/53224564-2d14e980-36b0-11e9-85f4-46304513b18d.png">

[iOS] [Fixed] - Remove compiler warning
Pull Request resolved: https://github.com/facebook/react-native/pull/23588

Differential Revision: D14181748

Pulled By: cpojer

fbshipit-source-id: 8b633e7cdb7b3b8029f4145a1155e540ac516191