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>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
(cherry picked from commit dd6d82112a10796dd5aa0f9e9c76ec8e77e7e211)

package/kernel/gpio-button-hotplug/src/gpio-button-hotplug.c

@@ -28,6 +28,7 @@
 #include <linux/of_gpio.h>
 #include <linux/of_irq.h>
 #include <linux/gpio_keys.h>
+#include <linux/gpio/consumer.h>
 
 #define BH_SKB_SIZE	2048