pinctrl: fix "warning: 'struct pinctrl_dev' declared inside parameter list"

when pinctl subsystem is not selected, when compiling drivers including
the include/linux/pinctrl/pinctrl.h, we will get the warning as below:
In file included from include/linux/pinctrl/pinmux.h:17,
                 from drivers/tty/serial/sirfsoc_uart.c:25:
include/linux/pinctrl/pinctrl.h:126: warning: 'struct pinctrl_dev'
		declared inside parameter list
include/linux/pinctrl/pinctrl.h:126: warning: its scope is only this
      definition or declaration, which is probably not what you want
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

include/linux/pinctrl/pinctrl.h

@@ -121,6 +121,7 @@ extern const char *pinctrl_dev_get_name(struct pinctrl_dev *pctldev);
 extern void *pinctrl_dev_get_drvdata(struct pinctrl_dev *pctldev);
 #else
 
+struct pinctrl_dev;
 
 /* Sufficiently stupid default function when pinctrl is not in use */
 static inline bool pin_is_valid(struct pinctrl_dev *pctldev, int pin)