Static instead of global variable for the include check

I have two issues with $_LIGHTBOX2_INCLUDE being a global variable:
1. Globals are bad, and duplicating the include check on the caller side is not optimal either.
2. Lightbox2's usage of globals doesn't work at my place, and the files are included multiple times. Possibly some globals restriction from my standard Ubuntu/Apache/PHP settings.

I propose the attached patch that makes the code a bit cleaner and additionally makes Lightbox2 work on my system. Other than this little issue, Lightbox2 is working great - thanks a lot for this neat module, it's the image display solution I always had searched for :D