shell/Kconfig: fix SHELL_SHUTDOWN_ON_EXIT dependency
shell/Kconfig: fix SHELL_SHUTDOWN_ON_EXIT dependency

Symbol `CPU_NATIVE` does not exist within Kconfig.

ant9000/RIOT,ant9000/RIOT,RIOT-OS/RIOT,miri64/RIOT,OlegHahm/RIOT,kYc0o/RIOT,kaspar030/RIOT,OTAkeys/RIOT,authmillenon/RIOT,authmillenon/RIOT,kaspar030/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,jasonatran/RIOT,jasonatran/RIOT,OlegHahm/RIOT,ant9000/RIOT,OTAkeys/RIOT,jasonatran/RIOT,kYc0o/RIOT,miri64/RIOT,OTAkeys/RIOT,miri64/RIOT,kaspar030/RIOT,OTAkeys/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,ant9000/RIOT,kaspar030/RIOT,OlegHahm/RIOT,authmillenon/RIOT,kYc0o/RIOT,ant9000/RIOT,jasonatran/RIOT,authmillenon/RIOT,kYc0o/RIOT,OlegHahm/RIOT,miri64/RIOT,kaspar030/RIOT,miri64/RIOT,OlegHahm/RIOT,RIOT-OS/RIOT