What means error LNK2019 and how to resolve it in QT?

Question

Maybe this error is invoking because of my wrong include statements... Error messages:

mainwindow.obj:-1: error: LNK2019: unresolved external symbol "public: __thiscall MatrixGetter::MatrixGetter(void)" (??0MatrixGetter@@QAE@XZ) referenced in function "private: void __thiscall MainWindow::on_pushButton_clicked(void)" (?on_pushButton_clicked@MainWindow@@AAEXXZ)

mainwindow.obj:-1: error: LNK2019: unresolved external symbol "public: void __thiscall MatrixGetter::getCountries(void)" (?getCountries@MatrixGetter@@QAEXXZ) referenced in function "private: void __thiscall MainWindow::on_pushButton_clicked(void)" (?on_pushButton_clicked@MainWindow@@AAEXXZ) 

debug\MdpProject.exe:-1: error: LNK1120: 2 unresolved externals

MdpProject.pro file

######################################################################
# Automatically generated by qmake (3.0) ?? ??? 27 01:48:55 2014
######################################################################

TEMPLATE = app
TARGET = MdpProject
INCLUDEPATH += .
QT += sql
# Input
HEADERS += dialog.h mainwindow.h matrixgetter.h matrixgetterdao.h
FORMS += dialog.ui mainwindow.ui
SOURCES += dialog.cpp \
           main.cpp \
           mainwindow.cpp \
           matrixgetter.cpp \
           matrixgetterdao.cpp

MainWindow.h class:

    #ifndef MAINWINDOW_H
    #define MAINWINDOW_H

    #include <QMainWindow>
    #include "matrixgetter.h"

    namespace Ui {
    class MainWindow;
    }

    class MainWindow : public QMainWindow
    {
        Q_OBJECT


    public:
        explicit MainWindow(QWidget *parent = 0);
        ~MainWindow();

    private slots:
        void on_pushButton_clicked();

    private:
        Ui::MainWindow *ui;
    };

    #endif // MAINWINDOW_H

MainWindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "dialog.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    ui->progressBar->setValue(100);

}

QString prop;
QString country;
MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::on_pushButton_clicked()
{

    Dialog *d = new Dialog();
//    MatrixGetter* mgetter = new MatrixGetter();
//    mgetter->getCountries();
    d -> show();
}

If I uncomment these lines, all will be fine. MatrixGetter has one empty method getCountries():

MatrixGetter.h

#ifndef MATRIXGETTER_H
#define MATRIXGETTER_H
namespace Ui {
    class MatrixGetter;
}

class MatrixGetter
{
public:
    MatrixGetter();
    void getCountries();
};

#endif // MATRIXGETTER_H

MatrixGetter.cpp

#include "matrixgetter.h"


MatrixGetter::MatrixGetter()
{}

void MatrixGetter::getCountries(){}