Add missing include.

Summary:
Fixes build.  I don't know why this didn't fail earlier, but it's
failing in the open source builds.

Fixes #1058

Test Plan: Compiles.

Reviewers: jdelong, seanc, mwilliams, sgolemon, ptarjan, bertrand

Reviewed By: seanc

CC: ps, hphp-diffs@lists

Differential Revision: https://phabricator.fb.com/D970243