staging: iio: adt7316: Fix build errors when GPIOLIB is not set

On x86_64 when GPIOLIB is not set the following build errors
are seen:

drivers/staging/iio/addac/adt7316.c:947:3: error: implicit declaration of function 'gpiod_set_value' [-Werror=implicit-function-declaration]
drivers/staging/iio/addac/adt7316.c:1805:2: error: implicit declaration of function 'irqd_get_trigger_type' [-Werror=implicit-function-declaration]

These functions are provided by the <linux/gpio/consumer.h>
and <linux/irq.h> headers, so include them to fix these
build errors.

While at it, remove <linux/gpio.h> as this driver is a GPIO
consumer and not a GPIO driver.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>