osmjs: enable absolute paths for the .js file

When specifying an absolute path, I got:
> $ /home/simon/src/osmium/osmjs/osmjs -2 -m -l sparsetable -i /home/simon/src/osmium/osmjs/js/osm2shape.js
> Could not find include file /home/simon/src/osmium/osmjs/js/osm2shape.js in search path (.:js:/home/simon/.osmjs:/usr/local/share/osmjs:/usr/share/osmjs)