kconfig: fix the rule of mainmenu_stmt symbol

The rule of mainmenu_stmt does not have debug print of zconf_lineno(),
but if it had, it would print a wrong line number for the same reason
as commit b2d00d7c ("kconfig: fix line numbers for if-entries in
menu tree").

The mainmenu_stmt does not need to eat following empty lines because
they are reduced to common_stmt.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>