sys/ztimer: add ztimer_periph_rtt module conflict warning
sys/ztimer: add ztimer_periph_rtt module conflict warning

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