Qt Unresolved external symbol LNK2019

Qt Unresolved external symbol LNK2019

I know that this topic has been discussed, but i really can't find a
solution for my problem. I want to read data from an XML file with
QDomDocument and i get errors of this type:
main.obj:-1: error: LNK2019: unresolved external symbol
"__declspec(dllimport) public: class QDomElement __cdecl
QDomNode::toElement(void)const "
(__imp_?toElement@QDomNode@@QEBA?AVQDomElement@@XZ) referenced in function
"void __cdecl ListElements(class QDomElement,class QString,class QString)"
(?ListElements@@YAXVQDomElement@@VQString@@1@Z)
I included
#include <QCoreApplication>
#include <QtXml/QtXml>
#include <QDebug>
and i also added Qt += xml to .pro file