mfd: max8997: 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/max8997.c

@@ -24,6 +24,7 @@
 #include <linux/err.h>
 #include <linux/slab.h>
 #include <linux/i2c.h>
+#include <linux/of.h>
 #include <linux/of_irq.h>
 #include <linux/interrupt.h>
 #include <linux/pm_runtime.h>