iio: adc: twl4030-madc: add missing of.h include

The driver is using of_device_id/of_match_ptr() and therefore needs
to include of.h header. We used to get this definition indirectly via
inclusion of matrix_keypad.h from twl.h, but we are cleaning up
matrix_keypad.h from unnecessary includes.
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20220927154611.3330871-3-dmitry.torokhov@gmail.comSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/iio/adc/twl4030-madc.c

@@ -30,6 +30,7 @@
 #include <linux/types.h>
 #include <linux/gfp.h>
 #include <linux/err.h>
+#include <linux/of.h>
 #include <linux/regulator/consumer.h>
 
 #include <linux/iio/iio.h>