sys/malloc_thread_safe: mark functions "used"
sys/malloc_thread_safe: mark functions "used"

Fixes LTO.

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