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 <[e-mail]>
Acked-by: Jonathan Cameron <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Dmitry Torokhov <[e-mail]>

drivers/iio/adc/twl4030-madc.c

···
 #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>