gpio: adp5588-gpio: add i2c forward declaration

Some ADP5588 functions take a pointer to an i2c_client, but if the i2c
header doesn't happen to be included first, we hit the standard "struct
declared inside parameter list" warnings from gcc.  So add a simple
forward decl of the i2c_client struct.
Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/i2c/adp5588.h

@@ -140,6 +140,8 @@ struct adp5588_kpad_platform_data {
 	const struct adp5588_gpio_platform_data *gpio_data;
 };
 
+struct i2c_client; /* forward declaration */
+
 struct adp5588_gpio_platform_data {
 	int gpio_start;		/* GPIO Chip base # */
 	unsigned irq_base;	/* interrupt base # */