iio: adc: mcp3422: 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: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/adc/mcp3422.c

@@ -20,6 +20,7 @@
 #include <linux/module.h>
 #include <linux/delay.h>
 #include <linux/sysfs.h>
+#include <linux/of.h>
 
 #include <linux/iio/iio.h>
 #include <linux/iio/sysfs.h>