ASoC: atmel: 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/atmel/sam9g20_wm8731.c

@@ -37,6 +37,7 @@
 #include <linux/interrupt.h>
 #include <linux/platform_device.h>
 #include <linux/i2c.h>
+#include <linux/of.h>
 
 #include <linux/atmel-ssc.h>