sys/string_utils: fix check for native
sys/string_utils: fix check for native

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