power: supply: max77976: add EXTCON dependency

power: supply: max77976: add EXTCON dependency

max8971 is written to work with or without extcon, but it fails
to link when built-in when the extcon subsystem is in a loadable
module:

x86_64-linux-ld: drivers/power/supply/max8971_charger.o: in function `max8971_probe':
max8971_charger.c:(.text+0x2ab): undefined reference to `extcon_find_edev_by_node'

Add the appropriate dependency that allows the working cases but
prevents the link failure.

Fixes: 60cd40eee4f4 ("power: supply: Add support for Maxim MAX8971 charger")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20250503140431.438727-1-arnd@kernel.org
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>