cpu/avr8_common: add strings.h for compatibility
cpu/avr8_common: add strings.h for compatibility

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