pinctrl: add "struct seq_file;" to pinconf.h

This allows one to include pinconf.h without having to include other
headers first.
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

include/linux/pinctrl/pinconf.h

@@ -15,6 +15,7 @@
 #ifdef CONFIG_PINCONF
 
 struct pinctrl_dev;
+struct seq_file;
 
 /**
  * struct pinconf_ops - pin config operations, to be implemented by