extcon: link devres into core module

Splitting the resource-managed functions into a separate module
means that the extcon core now fails to build because the internal
"extcon_dev_allocate" symbol is not exported:

ERROR: extcon_dev_allocate [drivers/extcon/devres.ko] undefined!

My guess is that the intention was not to have two separate
modules (which could be fixed by adding an export, plus the
normal MODULE_AUTHOR/MODULE_LICENSE/... fields), but have two
source files in the same module.

This fixes the Makefile accordingly, making the name of the
module extcon_core.ko, which is created from building both
extcon.c and devres.c.

Fixes: b225d00f ("extcon: Split out the resource-managed functions from extcon core")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>