regulator: bd718x7: Drop unused include

This driver does not use any symbols from <linux/gpio.h>
so just drop the include.

Cc: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-By: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Signed-off-by: Mark Brown <broonie@kernel.org>