iio: adc: ad7780: Add missing GPIOLIB dependency

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>