mfd: max8907: 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/max8907.c

@@ -17,6 +17,7 @@
 #include <linux/mfd/core.h>
 #include <linux/mfd/max8907.h>
 #include <linux/module.h>
+#include <linux/of.h>
 #include <linux/of_device.h>
 #include <linux/regmap.h>
 #include <linux/slab.h>