iio: adc: ti-adc081c: 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/ti-adc081c.c

@@ -9,6 +9,7 @@
 #include <linux/err.h>
 #include <linux/i2c.h>
 #include <linux/module.h>
+#include <linux/of.h>
 
 #include <linux/iio/iio.h>
 #include <linux/regulator/consumer.h>