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>