debian-stretch-build-dist: add python3-nwdiag
Recent versions of osmo-gsm-manuals need python3-nwdiag instead of the
python2 version. Install that too to successfully run the configure
script.
Related: OS#5347
Related: https://jenkins.osmocom.org/jenkins/job/Osmocom-release-tarballs/920/console
Change-Id: I127bd3855d44ccc68a5b0c4d83147fb470cc10c9
