We already create IMPORTED targets for the qtbase tools
since qtbase/189a5d8af12788a30121210066de647a6dc3a167

Change-Id: I89a1c59b4e14f0f5bdc71dfca0165e625c779715
Reviewed-by: Brad King <brad.king@kitware.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>