kconfig: use parent->dep as the parentdep of 'menu'

In menu_finalize(), the dependency of a menu entry is propagated
downwards.

For the 'menu', parent->dep and parent->prompt->visible.expr have
the same expression. Both accumulate the 'depends on' of itself and
upper menu entries.

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