regulator: bd718x7: add missing linux/of.h inclusion

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>

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Signed-off-by: Mark Brown <broonie@kernel.org>