build: work around automake-1.15 & intltool complaining about translations in build dir

build: work around automake-1.15 & intltool complaining about translations in build dir

make distcheck runs a build in dist directory subtirs and then runs intltool -m
which in turn complains about translations in a built file:

  The following files contain translations and are currently not in use. Please
  consider adding these to the POTFILES.in file, located in the po/ directory.

  sub/policy/org.freedesktop.NetworkManager.policy.in

The issue is reported to intltool upstream already:
https://bugs.launchpad.net/intltool/+bug/1117944