kconfig: qconf: convert the last old connection syntax to Qt5 style

kconfig: qconf: convert the last old connection syntax to Qt5 style

Commit a2574c12df0d ("kconfig: qconf: convert to Qt5 new signal/slot
connection syntax") converted most of the old string-based connections,
but one more instance still remains. Convert it to the new style.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>