js2c: fix module id generation on windows

Fix a regression that was introduced in commit 2db758c5 ("iojs: introduce
internal modules") where the computed id for "config.gypi" on Windows
was not "config" but an empty string.

With an empty string, the build succeeds but the binary is unusable:
startup.processConfig() in src/node.js chokes on the missing .config
property.

PR-URL: https://github.com/iojs/io.js/pull/1281


Reviewed-By: Vladimir Kurchatkin <vladimir.kurchatkin@gmail.com>