Fix singlefile build target

This PR makes a small fix to the singlefile build target.

After testing the final singlefile build in-browser and encountering a bunch of undefined variable errors, I realized that both the worker and non-worker source files need to be included within the same pdfjsWrapper closure to have access to the same global variables.

This requires only a small, simple change to the 'bundle' target. Now, instead of separately building pdf.js and pdf.worker.js then combining them to pdf.combined.js, the SINGLE_FILE flag causes the bundle target to put all source files into the same pdf.js, then the singlefile target moves pdf.js to pdf.combined.js.