Add missing include for flock()
Add missing include for flock()

Summary: flock() expects sys/file.h which on many platforms
is getting included as a side-effect, but on at least one
this isn't the case.

Add the explicit include to fix builds on those platforms.

Closes #232

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Unrelated runtime failures.

Reviewed By: @JoelMarcey

Differential Revision: D2196306

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