kleopatra: Set custom KLEOPATRA_VERSION_STRING for Gpg4win

* src/Makefile.am (gpg4win_pkg_kleopatra_configure): Don't set
RELEASE_SERVICE_VERSION. Set KLEOPATRA_VERSION_STRING.

Until recently RELEASE_SERVICE_VERSION was appended in parenthesis to
the normal Kleopatra version number. Now we set KLEOPATRA_VERSION_STRING
to override the full version string displayed by Kleopatra.

• GnuPG-bug-id: T7666