sys/xtimer/kconfig: fix dependencies
sys/xtimer/kconfig: fix dependencies

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