tidy a rogue include
tidy a rogue include

Summary:
Duplicate of line 22. Since the included header contains #pragma once it was doing nothing.

If it hadn't been a duplicate it would have been a problem because it was included within either a `namespace folly` or `namespace std...` block.

Reviewed By: yfeldblum

Differential Revision: D8031245

fbshipit-source-id: 4486859a11939a61d634c5603243785e6ec5a36c

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly