kconfig: qconf: use if_changed for qconf.moc rule

Regenerate qconf.moc when the moc command is changed.

This also allows 'make mrproper' to clean it up. Previously, it was
not cleaned up because 'clean-files += qconf.moc' was missing.
Now 'make mrproper' correctly cleans it up because files listed in
'targets' are cleaned.

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