openbsd: fix platform build

R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/21705003

Patch from Ben Noordhuis <info@bnoordhuis.nl>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16122 ce2b1a6d-e550-0410-aec6-3dcde31c8c00