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>

scripts/kconfig/lxdialog/dialog.h

@@ -221,7 +221,6 @@ int dialog_menu(const char *title, const char *prompt,
 		const void *selected, int *s_scroll);
 int dialog_checklist(const char *title, const char *prompt, int height,
 		     int width, int list_height);
-extern char dialog_input_result[];
 int dialog_inputbox(const char *title, const char *prompt, int height,
 		    int width, const char *init);