app_main: Function should return void not int

Thanks to @kolban : http://esp32.com/viewtopic.php?f=13&t=450&p=2030

main/main.c

@@ -11,7 +11,7 @@ esp_err_t event_handler(void *ctx, system_event_t *event)
     return ESP_OK;
 }
 
-int app_main(void)
+void app_main(void)
 {
     nvs_flash_init();
     system_init();