`YGNode`: one byte of private storage
`YGNode`: one byte of private storage

Summary:
Adds one byte of private storage to `YGNode`, intended to be used by Yoga itself.

This is in previously unused alignment space, and won’t cause more memory to be allocated.

Reviewed By: SidharthGuglani

Differential Revision: D15296732

fbshipit-source-id: 3caf0a3cd506e4e324e51c31869c69be5781d476

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