gpio-button-hotplug: fix 4.19 build breakage on malta/be64

While testing 4.19 build on malta/be64, I've encountered following
error:

 gpio-button-hotplug/gpio-button-hotplug.c:529:18: error: implicit
 declaration of function 'gpio_to_desc'

which is caused by the missing include fixed by this patch.

Signed-off-by: Petr Štetiar <ynezz@true.cz>