[hack] Fix Windows build
[hack] Fix Windows build

Summary:
I broke this in D18372942, but it was not caught in sandcastle because we don't
try to build for Windows there. Found by looking at AppVeyor build failures on
GitHub.

Reviewed By: dsainati1

Differential Revision: D18429191

fbshipit-source-id: 47b312ec188b68ad34311c7a904d8e25eca232a3

samwgoldman/flow,facebook/flow,facebook/flow,samwgoldman/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,samwgoldman/flow,mroch/flow,mroch/flow,samwgoldman/flow,samwgoldman/flow,samwgoldman/flow,mroch/flow,samwgoldman/flow,facebook/flow