button: add missing ADC dependency for BUTTON_ADC

The BUTTON_ADC symbol guards the compilation of button-adc driver whose
name very well makes it explicit that it requires ADC support to be
enabled.

Fix build issue of button-adc driver when ADC support isn't enabled by
making sure it cannot be built without ADC support.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
diff --git a/drivers/button/Kconfig b/drivers/button/Kconfig
index 097b05f..3918b05 100644
--- a/drivers/button/Kconfig
+++ b/drivers/button/Kconfig
@@ -12,6 +12,7 @@
 config BUTTON_ADC
 	bool "Button adc"
 	depends on BUTTON
+	depends on ADC
 	help
 	  Enable support for buttons which are connected to Analog to Digital
 	  Converter device. The ADC driver must use driver model. Buttons are