Add this block to your main.cpp:
extern "C" {
void app_main(void);
}
Example
#include <stdio.h>
#include "freertos/FreeRTOS.h"
#include "freertos/task.h"
#include "esp_system.h"
#include "esp_spi_flash.h"
extern "C" {
void app_main(void);
}
void app_main()
{
printf("Hello PlatformIO!\n");
}