make sure to zero window flag for login window

src/widget/loginscreen.cpp

@@ -37,7 +37,7 @@ LoginScreen::LoginScreen(QWidget *parent) :
     ui->setupUi(this);
 
     // permanently disables maximize button https://github.com/tux3/qTox/issues/1973
-    this->setWindowFlags(windowFlags() ^ Qt::WindowMaximizeButtonHint);
+    this->setWindowFlags(windowFlags() &! Qt::WindowMaximizeButtonHint);
     this->setFixedSize(this->size());
 
     connect(&quitShortcut, &QShortcut::activated, this, &LoginScreen::close);