Issue with symlinked modules

This diff causes npm to skip chown'ing symlinked node modules as modules installed with npm -g that contain a "bin" entry in package.json should already have correct permissions.

Example: 
sudo npm install -g node-gyp
cd /tmp
npm link node-gyp
npm ERR! Error: EPERM, chmod '/tmp/node_modules/node-gyp/bin/node-gyp.js'