kconfig: gconf: fix behavior of a menu under a symbol in split view

A menu can be created under a symbol.

[Example]

  menu "outer menu"

  config A
          bool "A"

  menu "inner menu"
          depends on A

  config B
          bool "B"

  endmenu

  endmenu

After being re-parented by menu_finalize(), the menu tree is structured
like follows:

  menu "outer menu"
  \-- A
      \-- menu "inner menu"
          \-- B

In split view, the symbol A is shown in the right pane, so all of its
descendants must also be shown there. This has never worked correctly.

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