Add Ubuntu's drm_fix patch

[SergeyTsalkov]

Ubuntu (and I think Debian too) add a drm_fix patch to their copy of pdftk. This makes it work on PDFs that have an owner password set to enable certain PDF features, but are not encrypted. I'm working on a project where PDFs come in from weird sources, so this is quite necessary.

You can see the patch here: drm_fix.txt

Or get it from Ubuntu here: http://packages.ubuntu.com/trusty/pdftk