GN: Add make_core_generated->m..private_script dep

This is needed to build the generated bindings.

R=brettw@chromium.org

Review URL: https://codereview.chromium.org/359693002

git-svn-id: svn://svn.chromium.org/blink/trunk@177159 bbb929c8-8fbe-4397-9dbb-9b2b20218538