kconfig:lxdialog: remove duplicate code

dialog.h has two line the same below:
extern char dialog_input_result[];
This patch remove one of them.

Signed-off-by: Wang YanQing <udknight@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>