mfd: max77686: Include linux/of.h header

'of_match_ptr' is defined in linux/of.h. Include it explicitly to
avoid breakage in the future.
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/mfd/max77686.c

@@ -31,6 +31,7 @@
 #include <linux/mfd/max77686.h>
 #include <linux/mfd/max77686-private.h>
 #include <linux/err.h>
+#include <linux/of.h>
 
 #define I2C_ADDR_RTC	(0x0C >> 1)