ADC ATMega8 ADC_Vect warning

#define F_CPU 1000000UL

#include <avr/io.h>

#include <stdlib.h>

#include <avr/interrupt.h>

#include <avr/pgmspace.h>

#include <util/delay.h>

#include "lcd.h"

int main(void)

{

  ADCSRA |= 1<<ADPS2;

  ADMUX |= 1<<ADLAR;

  ADMUX |= 1<<REFS0;

  ADCSRA |= 1<<ADIE; 

  ADCSRA |= 1<<ADEN;

  sei();

  ADCSRA |= 1<<ADSC;

        lcd_init(LCD_DISP_ON);

             while(1)

             {  

             }

}

ISR(ADC_Vect)

{

    char adcResult[4];

    itoa(ADCH, adcResult, 10); 

    lcd_clrscr();

    lcd_puts(adcResult);

    ADCSRA |= 1<<ADSC; 

}