Use the portability header for malloc_usable_size in small_vector.h
Use the portability header for malloc_usable_size in small_vector.h

Summary: `small_vector.h` had it's own thing being defined inline, and it was breaking the OSX build of HHVM, so have it use the portability header instead.

Reviewed By: yfeldblum

Differential Revision: D3232885

fb-gh-sync-id: 2078a54d1ec9700e81e0a9f4baed8e2d623fa5f3
fbshipit-source-id: 2078a54d1ec9700e81e0a9f4baed8e2d623fa5f3

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