mfd: max77693: 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: Lee Jones <lee.jones@linaro.org>

drivers/mfd/max77693.c

@@ -28,6 +28,7 @@
 #include <linux/i2c.h>
 #include <linux/err.h>
 #include <linux/interrupt.h>
+#include <linux/of.h>
 #include <linux/pm_runtime.h>
 #include <linux/mutex.h>
 #include <linux/mfd/core.h>