ASoC: pcm1681: Include linux/of.h header

'of_match_ptr' is defined in linux/of.h. Include it explicitly.
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>

sound/soc/codecs/pcm1681.c

@@ -21,6 +21,7 @@
 #include <linux/gpio.h>
 #include <linux/i2c.h>
 #include <linux/regmap.h>
+#include <linux/of.h>
 #include <linux/of_device.h>
 #include <linux/of_gpio.h>
 #include <sound/pcm.h>