extcon: class: Remove unnecessary extern declaration

This patch remove unnecessary extern declaration (extcon_set_state).
checkpatch found this coding style issue.
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Myungjoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/extcon/extcon-class.c

@@ -129,7 +129,6 @@ static ssize_t state_show(struct device *dev, struct device_attribute *attr,
 	return count;
 }
 
-int extcon_set_state(struct extcon_dev *edev, u32 state);
 static ssize_t state_store(struct device *dev, struct device_attribute *attr,
 			   const char *buf, size_t count)
 {