Add build_popper71.patch to fix build with poppler 0.71

Ubuntu 19.04 has started the poppler 0.71 transition

https://launchpad.net/ubuntu/+source/popplerkit.framework/0.0.20051227svn-7.1ubuntu1

https://people.canonical.com/~ubuntu-archive/transitions/html/poppler.html

cc/ @pino