gpio: fix warning of 'struct gpio_chip' declaration

The struct gpio_chip is only defined inside #ifdef CONFIG_GPIOLIB,
but it's referenced by gpiochip_add_pin_range() and
gpiochip_remove_pin_ranges() which are outside #ifdef CONFIG_GPIOLIB.
Thus, we see the following warning when building blackfin image, where
GPIOLIB is not required.

  CC      arch/blackfin/kernel/bfin_gpio.o
  CC      init/version.o
In file included from arch/blackfin/include/asm/gpio.h:321,
                 from arch/blackfin/kernel/bfin_gpio.c:15:
include/asm-generic/gpio.h:298: warning: 'struct gpio_chip' declared inside parameter list
include/asm-generic/gpio.h:298: warning: its scope is only this definition or declaration, which is probably not what you want
include/asm-generic/gpio.h:304: warning: 'struct gpio_chip' declared inside parameter list

Move pinctrl trunk into #ifdef CONFIG_GPIOLIB to fix the warning,
since it appears that pinctrl gpio range support depends on GPIOLIB.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>