platform/x86: yt2-1380: add CONFIG_EXTCON dependency

This driver uses the extcon subsystem and fails to build
when it cannot call into that subsystem:

x86_64-linux-ld: vmlinux.o: in function `yt2_1380_fc_worker':
lenovo-yoga-tab2-pro-1380-fastcharger.c:(.text+0xa9d819): undefined reference to `extcon_get_state'
x86_64-linux-ld: lenovo-yoga-tab2-pro-1380-fastcharger.c:(.text+0xa9d853): undefined reference to `extcon_get_state'
x86_64-linux-ld: vmlinux.o: in function `yt2_1380_fc_serdev_probe':
lenovo-yoga-tab2-pro-1380-fastcharger.c:(.text+0xa9da22): undefined reference to `extcon_get_extcon_dev'
x86_64-linux-ld: lenovo-yoga-tab2-pro-1380-fastcharger.c:(.text+0xa9dc0c): undefined reference to `devm_extcon_register_notifier_all'

Add a Kconfig dependency to make it it always builds correctly.

Fixes: b2ed33e ("platform/x86: Add lenovo-yoga-tab2-pro-1380-fastcharger driver")
Signed-off-by: Arnd Bergmann <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Reviewed-by: Hans de Goede <[email protected]>
Signed-off-by: Hans de Goede <[email protected]>

Author: arndb

drivers/platform/x86/Kconfig

@@ -136,6 +136,7 @@ config YOGABOOK
 config YT2_1380
 	tristate "Lenovo Yoga Tablet 2 1380 fast charge driver"
 	depends on SERIAL_DEV_BUS
+	depends on EXTCON
 	depends on ACPI
 	help
 	  Say Y here to enable support for the custom fast charging protocol