QT undefined reference errors when trying to compile

I added a class IcecastServer to my QT-project, added the header-file to the pro file and added some code. Everytime I compile it the following errors occur:

release/icecastserver.o:icecastserver.cpp:(.text+0x39): undefined reference to _imp___ZN10QTcpServerC1EP7QObject' release/icecastserver.o:icecastserver.cpp:(.text+0x50): undefined reference toimpZN12QHostAddressC1ENS_14SpecialAddressE' release/icecastserver.o:icecastserver.cpp:(.text+0x68): undefined reference to _imp___ZN10QTcpServer6listenERK12QHostAddresst' release/icecastserver.o:icecastserver.cpp:(.text+0x73): undefined reference to_imp_ZN12QHostAddressD1Ev' release/icecastserver.o:icecastserver.cpp:(.text+0x9d): undefined reference to _imp___ZNK10QTcpServer11errorStringEv' release/icecastserver.o:icecastserver.cpp:(.text+0x3d4): undefined reference toimpZN12QHostAddressD1Ev' release/icecastserver.o:icecastserver.cpp:(.text+0x4bd): undefined reference to _imp___ZN10QTcpServerC1EP7QObject' release / icecastserver.o: icecastserver.cpp :(. text + 0x4d4): undefined ссылка на _imp _ZN12QHostAddressC1ENS_14SpecialAddressE ' release / icecastserver.o: icecastserver.cpp :(. text + 0x4ec): undefined ссылка на _imp ___ ZN10QTcpServer6listenERK12QHostAddresst ' release / icecastserver.o: icecastserver.cpp :(. text + 0x4f7): undefined ссылка на imp ZN12QHostAddressD1Ev ' release / icecastserver.o: icecastserver.cpp :(. text + 0x521): undefined ссылка на _imp ___ ZNK10QTcpServer11errorStringEv ' release / icecastserver.o: icecastserver.cpp :(. text + 0x858): undefined ссылка на _imp _ZN12QHostAddressD1Ev '

Что я делаю не так?

Это заголовочный файл:

#ifndef ICECASTSERVER_H
#define ICECASTSERVER_H

#include <QObject>

QT_BEGIN_NAMESPACE
class QTcpServer;
QT_END_NAMESPACE

class IcecastServer : public QObject
{
    Q_OBJECT
public:
    explicit IcecastServer(QObject *parent = 0);

signals:

public slots:

private:
    QTcpServer *tcpServer;
};

#endif // ICECASTSERVER_H

Это исходный файл:

#include "icecastserver.h"
#include "QDebug"
#include <QtNetwork/QTcpServer>
#include <QtGui>

IcecastServer::IcecastServer(QObject *parent) :
    QObject(parent)
{

    tcpServer = new QTcpServer(this);
    //tcpServer->listen(QHostAddress::Any,8000);

    if (!tcpServer->listen()){
        QMessageBox::critical(NULL, tr("Fortune Server"), tr("Unable to start the server: %1.").arg(tcpServer->errorString()));

         return;
    }

}