No match for 'operator<<'

#include <iostream>
#include "stonewt.h"
 
using namespace std;
 
void display(const Stonewt & st, int n);
 
int main()
{
    Stonewt incognito = 275;
    Stonewt wolfe(285.7);
    Stonewt taft(21);
    cout << "The celebrity weighed ";
    cout << incognito;
    cout << "The detective weighed ";
    cout << wolfe;
    cout << "The President weighed ";
    cout << taft;
    incognito = 276.8;
    taft = 325;
    cout << "After dinner, the celebrity weighed ";
    cout << incognito;
    cout << "After dinner, the President weighed ";
    cout << taft;
    display(taft, 2);
    cout << "The wrestler weighed even more.\n";
    display(422, 2);
    cout << "No stone left unearned\n";
    return 0;
}
 
void display(const Stonewt & st, int n){
    for (int i = 0; i < n; i++){
        cout << "Wow! ";
        cout << st;
    }
}

#ifndef STONEWT_H_INCLUDED
#define STONEWT_H_INCLUDED
 
class Stonewt{
public:
    enum Mode {Stones, poundsint, poundsdou};
private:
    enum {Lbs_per_stn = 14};
    int stone;
    int poundsi;
    double pounds;
    Mode mode;
public:
    Stonewt(int lbs, Mode form = poundsint);
    Stonewt(double lbs);
    Stonewt();
    ~Stonewt();
    friend std::ostream & operator<<(std::ostream & os, Stonewt & s);
};
#endif // STONEWT_H_INCLUDED

#include <iostream>
using std::cout;
using std::endl;
#include "stonewt.h"
 
Stonewt::Stonewt(int lbs, Mode form){
    mode = form;
    if (form == poundsint){
        pounds = lbs;
        poundsi = int (lbs);
        stone = Lbs_per_stn * lbs;
    }
    else if (form == Stones) {
        stone = lbs;
        pounds = lbs / Lbs_per_stn;
        poundsi = lbs / Lbs_per_stn;
    }
}
 
Stonewt::Stonewt(double lbs){
    mode = poundsdou;
    pounds = lbs;
    poundsi = int (lbs);
    stone = Lbs_per_stn * lbs;
}
 
Stonewt::Stonewt(){
    mode = poundsdou;
    stone = pounds = poundsi = 0;
}
 
Stonewt::~Stonewt(){
 
}
 
std::ostream & operator<<(std::ostream & os, Stonewt & s){
    if (s.mode == Stonewt::Stones){
        os << "Weight in stones: " << s.stone << endl;
    }
    else if (s.mode == Stonewt::poundsint){
        os << "Weight in whole pounds: " << s.poundsi << endl;
    }
    else {
        os << "Weight in pounds: " << s.pounds << endl;
    }
    return os;
}

friend std::ostream & operator<<(std::ostream & os, const Stonewt & s);