Follow symlinks when building XPIs.

Hi!

I'm trying to get the webxray goggles addon from https://github.com/bwinton/webxray working, and one of the things that code tries to do is symlink the source into the addon directory.  (The source should be shared between the Firefox and Chrome addons, so putting a copy in each directory isn't the greatest idea, and I'm loathe to introduce an entire build step just to copy the source into each directory.)

This simple change allows the files to exist in the symlinked directory, and still be bundled in the xpi.  (Note: I've only tested it with `cfx run` so far.)

Thanks,
Blake.