Just for consistency and cleanliness, following commit
4b48aca411. The 'package/debian' directory
is going to be removed soon, anyway[1].
The 'RedHat' and 'openSUSE' directories in 'package' don't need such a
change, because they use their own desktop file (and they should also be
removed after the 2017.2.1 release).
[1] https://sourceforge.net/p/flightgear/flightgear/merge-requests/92/