gpiolib: provide a declaration of seq_file in gpio/driver.h

Fixes this build error:

In file included from include/asm-generic/gpio.h:13:0,
                 from include/linux/gpio.h:51,
                 from include/linux/of_gpio.h:20,
                 from arch/powerpc/sysdev/ppc4xx_gpio.c:29:
include/linux/gpio/driver.h:85:14: error: 'struct seq_file' declared inside=
 parameter list [-Werror]
include/linux/gpio/driver.h:85:14: error: its scope is only this definition=
 or declaration, which is probably not what you want [-Werror]
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

include/linux/gpio/driver.h

@@ -5,6 +5,7 @@
 
 struct device;
 struct gpio_desc;
+struct seq_file;
 
 /**
  * struct gpio_chip - abstract a GPIO controller