Fix cross-compilation in Makefile.

Fix r22531, I dropped a necessary pair of parens.

BUG=
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/410003003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22565 ce2b1a6d-e550-0410-aec6-3dcde31c8c00