mfd: tps6507x: 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/tps6507x.c

@@ -19,6 +19,7 @@
 #include <linux/init.h>
 #include <linux/slab.h>
 #include <linux/i2c.h>
+#include <linux/of.h>
 #include <linux/of_device.h>
 #include <linux/mfd/core.h>
 #include <linux/mfd/tps6507x.h>