asm-generic/gpio.h: add some forward decls of the device struct

After the recent commit a4177ee7, attempting to include asm-generic/gpio.h
in otherwise "slim" code results in ugly warnings like so:

  CC      arch/blackfin/kernel/bfin_gpio.o
In file included from arch/blackfin/include/asm/gpio.h:278,
                 from arch/blackfin/kernel/bfin_gpio.c:15:
include/asm-generic/gpio.h:193: warning:
‘struct device’ declared inside parameter list
its scope is only this definition or declaration, which is probably not what you want

So add simple C forward decls of the struct device to avoid these.
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

include/asm-generic/gpio.h

@@ -28,6 +28,7 @@ static inline int gpio_is_valid(int number)
 	return ((unsigned)number) < ARCH_NR_GPIOS;
 }
 
+struct device;
 struct seq_file;
 struct module;
 
@@ -181,6 +182,8 @@ static inline void gpio_set_value_cansleep(unsigned gpio, int value)
 
 #ifndef CONFIG_GPIO_SYSFS
 
+struct device;
+
 /* sysfs support is only available with gpiolib, where it's optional */
 
 static inline int gpio_export(unsigned gpio, bool direction_may_change)