kconfig: port qconf to work with Qt6 in addition to Qt5

Tested with Qt5 5.15 and Qt6 6.4. Note that earlier versions of Qt5
are no longer guaranteed to work.

Signed-off-by: Boris Kolpackov <boris@codesynthesis.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>