gpio: Fix build warnings on undefined struct pinctrl_dev

This fixes the warnings:

* include/linux/gpio.h:254:11: warning: 'struct pinctrl_dev' declared
  inside parameter list will not be visible outside of this definition
  or declaration
* include/linux/gpio/driver.h:602:11: warning: 'struct pinctrl_dev'
  declared inside parameter list will not be visible outside of this
  definition or declaration

Fixes: 78b99577b3 ("pinctrl: remove unused pin_is_valid()")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Enrico Weigelt <info@metux.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

include/linux/gpio.h

@@ -106,6 +106,7 @@ void devm_gpio_free(struct device *dev, unsigned int gpio);
 
 struct device;
 struct gpio_chip;
+struct pinctrl_dev;
 
 static inline bool gpio_is_valid(int number)
 {

include/linux/gpio/driver.h

@@ -590,6 +590,8 @@ void gpiochip_remove_pin_ranges(struct gpio_chip *chip);
 
 #else
 
+struct pinctrl_dev;
+
 static inline int
 gpiochip_add_pin_range(struct gpio_chip *chip, const char *pinctl_name,
 		       unsigned int gpio_offset, unsigned int pin_offset,