gpiolib: Add 'struct gpio_chip' forward declaration for !GPIOLIB case

With CONFIG_GPIOLIB=n, the 'struct gpio_chip' is not declared,
so the following pops up on PowerPC:

  cc1: warnings being treated as errors
  In file included from arch/powerpc/platforms/52xx/mpc52xx_common.c:19:
  include/linux/of_gpio.h:74: warning: 'struct gpio_chip' declared
                              inside parameter list
  include/linux/of_gpio.h:74: warning: its scope is only this definition
                              or declaration, which is probably not what
			      you want
  include/linux/of_gpio.h:75: warning: 'struct gpio_chip' declared
                              inside parameter list
  make[2]: *** [arch/powerpc/platforms/52xx/mpc52xx_common.o] Error 1

This patch fixes the issue by providing the proper forward declaration.
Signed-off-by: NAnton Vorontsov <cbouatmailru@gmail.com>
Signed-off-by: NGrant Likely <grant.likely@secretlab.ca>

include/linux/gpio.h

@@ -13,6 +13,7 @@
 #include <linux/errno.h>
 
 struct device;
+struct gpio_chip;
 
 /*
  * Some platforms don't support the GPIO programming interface.