Fix external snapshot compilation

TBR=yangguo@chromium.org
LOG=n
BUG=none

Review URL: https://codereview.chromium.org/341433005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22097 ce2b1a6d-e550-0410-aec6-3dcde31c8c00