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 <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/mfd/sm501.c

@@ -17,6 +17,7 @@
 #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>