Fix detection of private Qt6 headers

Fix detection of private Qt6 headers

The gui-private is a separate module. Private Qt6 GUI headers
are currently used only with the Windows backend. However they
are needed to port to Qt6 all functionality that was depending
on the -extras modules. For example, all code guarded by the
macro HAVE_QT5_X11_EXTRAS has to be audited and properly replaced
by Qt6 code. See for example 771f30e9 and 17b24cf4.