build fix for d8-readline for 15142

R=svenpanne@chromium.org
BUG=

Review URL: https://codereview.chromium.org/17028013

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15151 ce2b1a6d-e550-0410-aec6-3dcde31c8c00