mfd: sm501: Include the GPIO driver header

This driver creates a gpio chip so it needs to include the
appropriate header <linux/gpio/driver.h> explicitly rather
than implicitly.

Signed-off-by: Linus Walleij <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>

drivers/mfd/sm501.c

···
 #include <linux/platform_device.h>
 #include <linux/pci.h>
 #include <linux/platform_data/i2c-gpio.h>
+#include <linux/gpio/driver.h>
 #include <linux/gpio/machine.h>
 #include <linux/slab.h>