iio: dac: ltc1660: add missing select REGMAP_SPI in Kconfig

This driver makes use of regmap_spi, but does not select the required
module.
Add the missing 'select REGMAP_SPI'.

Fixes: 8316cebd ("iio: dac: add support for ltc1660")
Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Link: https://patch.msgid.link/20241003-ad2s1210-select-v1-7-4019453f8c33@gmail.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/dac/Kconfig

@@ -358,6 +358,7 @@ config LPC18XX_DAC
 config LTC1660
 	tristate "Linear Technology LTC1660/LTC1665 DAC SPI driver"
 	depends on SPI
+	select REGMAP_SPI
 	help
 	  Say yes here to build support for Linear Technology
 	  LTC1660 and LTC1665 Digital to Analog Converters.