core/xfa: make externc happy
core/xfa: make externc happy

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