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.
https://jenkins-iojs.nodesource.com/view/iojs/job/iojs+any-pr+multi/385/