gpio: rcar: Include linux/of.h header

'of_match_ptr' is defined in linux/of.h. Include it explicitly to
avoid build breakage in the future.
Signed-off-by: NSachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpio-rcar.c

@@ -22,6 +22,7 @@
 #include <linux/irq.h>
 #include <linux/irqdomain.h>
 #include <linux/module.h>
+#include <linux/of.h>
 #include <linux/pinctrl/consumer.h>
 #include <linux/platform_data/gpio-rcar.h>
 #include <linux/platform_device.h>