gpiolib: include gpio/consumer.h in of_gpio.h for desc_to_gpio()

Fixes this build error on sparc:

In file included from drivers/spi/spi.c:33:0:
include/linux/of_gpio.h: In function 'of_get_named_gpio_flags':
include/linux/of_gpio.h:93:3: error: implicit declaration of function 'desc_to_gpio' [-Werror=implicit-function-declaration]
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

include/linux/of_gpio.h

@@ -19,9 +19,9 @@
 #include <linux/errno.h>
 #include <linux/gpio.h>
 #include <linux/of.h>
+#include <linux/gpio/consumer.h>
 
 struct device_node;
-struct gpio_desc;
 
 /*
  * This is Linux-specific flags. By default controllers' and Linux' mapping