Update GpuIndexFlat.h (#1626)
Update GpuIndexFlat.h (#1626)

Summary:
visual Studio compile problem(undefine reference)

Pull Request resolved: https://github.com/facebookresearch/faiss/pull/1626

Reviewed By: beauby

Differential Revision: D25928072

Pulled By: wickedfoo

fbshipit-source-id: 464ad453f8f85a802e81d28d8f1a5fbae0b388f3

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