The single build rule does not work with the core-y syntax. [1]
Use the standard obj-y syntax.
[1]: https://lore.kernel.org/linux-kbuild/d57ba55f-20a3-b836-783d-b49c8a161b6e@xxxxxxxxxx/T/#m7bc402e1e038f00ebcf2e92ed7fcb8a52fc1ea44
Signed-off-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>