gpio: ath79: add missing header

Include mod_devicetable.h for struct of_device_id and its helpers.
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20240821121456.19553-2-brgl@bgdev.plSigned-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

drivers/gpio/gpio-ath79.c

@@ -11,6 +11,7 @@
 #include <linux/gpio/driver.h>
 #include <linux/interrupt.h>
 #include <linux/irq.h>
+#include <linux/mod_devicetable.h>
 #include <linux/module.h>
 #include <linux/of.h>
 #include <linux/platform_data/gpio-ath79.h>