gen-tarball: Add VERSION file to kleopatra snapshots

The first line is left empty because we don't have a useful version
number for the kleopatra snapshot. (We inject the version number of
Gpg4win, etc., when we build Gpg4win.) The second line contains the
full git commit ID which uniquely identifies the content of the
snapshot.

• GnuPG-bug-id: T7666