rebuild for stretch-backports

I prepared an upload for backports for https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=900851

Please consider merging the debian/stretch-backports branch into a branch of the same name on the main repo. I'm following DEP-14 https://dep-team.pages.debian.net/deps/dep14/

edit: if you want to, just create a branch of the same name on the repo, then i can remove the "WIP:" tag and you can merge using the salsa interface.

Thanks