regulator: bd718x7: add missing linux/of.h inclusion (deja-vu)

0-Day tests found compilation error on x86.
Driver won't compile on x86_64 as "of_match_ptr" is
not found. Add missing include <linux/of.h>

At some point this fix was lost. So re-apply it.
Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/regulator/bd718x7-regulator.c

@@ -9,6 +9,7 @@
 #include <linux/kernel.h>
 #include <linux/mfd/rohm-bd718x7.h>
 #include <linux/module.h>
+#include <linux/of.h>
 #include <linux/platform_device.h>
 #include <linux/regulator/driver.h>
 #include <linux/regulator/machine.h>