SYMBOL_CHANGED and MENU_CHANGED are used to update GUI frontends
when the symbol value is changed. These are used inconsistently:
SYMBOL_CHANGED in gconf.c and MENU_CHANGE in qconf.cc.

MENU_CHANGED works more properly when a symbol has multiple prompts
(although such code is not ideal).

[test code]

    config FOO
            bool "foo prompt 1"

    config FOO
            bool "foo prompt 2"

In gconfig, if one of the two checkboxes is clicked, only the first
one is toggled. In xconfig, the two checkboxes work in sync.

Replace SYMBOL_CHANGED in gconf.c with MENU_CHANGED to align with
the xconfig behavior.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>