add missing initializer_list include
add missing initializer_list include

Summary:
For whatever reason, this was only causing problems for whatever (out
of date) version of clang I had on my older mac laptop, but it is technically
appropriate to have this in this header file.

Reviewed By: farnz

Differential Revision: D3967938

fbshipit-source-id: a99e672405c48ae14536f34027a2978a18c43aff

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