Input: resistive-adc-touch - fix kconfig dependency on IIO_BUFFER

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

WARNING: unmet direct dependencies detected for IIO_BUFFER_CB
Depends on [n]: IIO [=y] && IIO_BUFFER [=n]
Selected by [y]:
- TOUCHSCREEN_ADC [=y] && !UML && INPUT [=y] && INPUT_TOUCHSCREEN [=y] && IIO [=y]

The reason is that TOUCHSCREEN_ADC selects IIO_BUFFER_CB without depending
on or selecting IIO_BUFFER while IIO_BUFFER_CB 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: aa132ffb6b0a ("input: touchscreen: resistive-adc-touch: add generic resistive ADC touchscreen")
Signed-off-by: Necip Fazil Yildiran <[e-mail]>
Acked-by: Jonathan Cameron <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Dmitry Torokhov <[e-mail]>

drivers/input/touchscreen/Kconfig

···
 config TOUCHSCREEN_ADC
 	tristate "Generic ADC based resistive touchscreen"
 	depends on IIO
+	select IIO_BUFFER
 	select IIO_BUFFER_CB
 	help
 	  Say Y here if you want to use the generic ADC