Input: gpio_keys - fix struct device declared inside parameter list

A struct device parameter is used in the enable and disable callbacks to
distinguish between different gpio_keys devices.

Platforms that don't use these callbacks may not include struct device
at all, as seen on arch/arm/mach-s3c2410/mach-n30.c

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>

include/linux/gpio_keys.h

@@ -1,6 +1,8 @@
 #ifndef _GPIO_KEYS_H
 #define _GPIO_KEYS_H
 
+struct device;
+
 struct gpio_keys_button {
 	/* Configuration parameters */
 	unsigned int code;	/* input event code (KEY_*, SW_*) */