Why do I get fpermissive for declaration the following function?

Under exercise 6.40 of C++ Primer, I am getting -fpermissive error for the following code:

#include <iostream>
#include <string>

using std::cin;
using std::cout;
using std::endl;
using std::string;

string make_plural(size_t ctr, const string &word, const string &ending = "s");

int main()
{
    string input;

    cout << "Success:" << make_plural(2, "Success", "es") << endl;
    cout << "Failure:" << make_plural(2, "Failure") << endl;

    return 0;
}

string make_plural(size_t ctr, const string &word, const string &ending = "s")
{
    return (ctr > 1) ? word + ending : word;
}

I am used to prototyping the function before the main function first before declaring the function after the main function. However, I am getting fpermissive flag from G++ compiler.

Would like to know why this happened.