Fix compilation with clang 5.1 on OS X.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/250873003

Patch from Benedikt Meurer <bmeurer@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20970 ce2b1a6d-e550-0410-aec6-3dcde31c8c00