Add zfs-import.target services in spec file

Add missing zfs-import.target to list of systemd services in zfs
RPM spec file.

Reviewed-by: Niklas Wagner <Skaro@Skaronator.com>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ralf Ertzinger <ralf@skytale.net>
Issue #6953
Closes #6955