Make gYGNodeDefaults const
Make gYGNodeDefaults const

Summary: At least one compiler seems to care about it.

Reviewed By: emilsjolander

Differential Revision: D5675518

fbshipit-source-id: 72a6e208263dde0b6bb46a78fedb2796d0e0a600

facebook/css-layout,facebook/css-layout,facebook/css-layout,facebook/yoga,facebook/yoga,facebook/css-layout,facebook/yoga,facebook/yoga,facebook/css-layout,facebook/yoga,facebook/yoga,facebook/css-layout,facebook/yoga,facebook/css-layout,facebook/yoga,facebook/css-layout,facebook/yoga,facebook/css-layout,facebook/yoga