async: silence different signedness comparison
async: silence different signedness comparison

Summary:
While building EdenFS on Windows, the compiler complains that the sides of the
comparison are using different types. Let's simply cast the right side to the
same type as the left side.

Reviewed By: iahs

Differential Revision: D24954601

fbshipit-source-id: 48ac1f863834d961b79d9ce0032521cf1bdf3ab8

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift