QApplication::desktop() returns a pointer to QDesktopWidget, not to
QWidget.

Fixes the following compiler error after a quick conversion with 'qt3to4',
which occured with g++ 3.4.6 and 4.1.2, but not anymore with 4.3.2.

scripts/kconfig/qconf.cc: In constructor 'ConfigMainWindow::ConfigMainWindow()':
scripts/kconfig/qconf.cc:1289: error: cannot convert 'QDesktopWidget*' to 'QWidget*' in initialization
Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>