Work around make distcheck issue (#778628)

The command 'make distcheck' runs a build in dist directory subdirs
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/gparted.desktop.in

See also upstream intltool issue bug report:

  intltool confused by separate build-dir
  https://bugs.launchpad.net/intltool/+bug/1117944

Bug 778628 - Work around automake-1.15 & intltool complaining about
             translations in build dir