src: hide InitializeICUDirectory symbol

Exporting it seems like an oversight.  It's not safe to call once
V8 is running so there doesn't seem to be a point in exporting it
to add-ons.  Un-export it.

PR-URL: https://github.com/nodejs/io.js/pull/1815


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>