charger-manager: Fix build break related to EXTCON

This patch select CONFIG_EXTCON to resolve below build break of
charger-manager because charger-manager use API of EXTCON subsystem.

drivers/built-in.o: In function `charger_manager_probe':
charger-manager.c:(.text+0x11d61a): undefined reference to `extcon_register_interest'
charger-manager.c:(.text+0x11d7b6): undefined reference to `extcon_unregister_interest'
drivers/built-in.o: In function `charger_manager_remove':
charger-manager.c:(.devexit.text+0x8f3): undefined reference to `extcon_unregister_interest'

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Myungjoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>