iio: light: fix kconfig dependency bug for VCNL4035

When VCNL4035 is enabled and IIO_BUFFER is disabled, it results in the
following Kbuild warning:

WARNING: unmet direct dependencies detected for IIO_TRIGGERED_BUFFER
  Depends on [n]: IIO [=y] && IIO_BUFFER [=n]
  Selected by [y]:
  - VCNL4035 [=y] && IIO [=y] && I2C [=y]

The reason is that VCNL4035 selects IIO_TRIGGERED_BUFFER without depending
on or selecting IIO_BUFFER while IIO_TRIGGERED_BUFFER depends on
IIO_BUFFER. This can also fail building the kernel.

Honor the kconfig dependency to remove unmet direct dependency warnings
and avoid any potential build failures.

Fixes: 55707294 ("iio: light: Add support for vishay vcnl4035")
Signed-off-by: Necip Fazil Yildiran <fazilyildiran@gmail.com>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=209883
Link: https://lore.kernel.org/r/20201102223523.572461-1-fazilyildiran@gmail.com
Cc: <stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/light/Kconfig

@@ -544,6 +544,7 @@ config VCNL4000
 
 config VCNL4035
 	tristate "VCNL4035 combined ALS and proximity sensor"
+	select IIO_BUFFER
 	select IIO_TRIGGERED_BUFFER
 	select REGMAP_I2C
 	depends on I2C