fix compilation issue with various compilers
fix compilation issue with various compilers

Summary:
This passed on the compilers in our internal CI, but failed
on some of the Travis and Circle CI hosts

Differential Revision: D7142088

fbshipit-source-id: 1792ab124092b9da5b5b56cec7266e2cc5fcb5b7

wez/watchman,nodakai/watchman,nodakai/watchman,wez/watchman,facebook/watchman,wez/watchman,wez/watchman,nodakai/watchman,wez/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,wez/watchman,nodakai/watchman,nodakai/watchman,wez/watchman,facebook/watchman,wez/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,wez/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman