kbuild: fix false-positive need-builtin calculation

The current implementation of need-builtin is false-positive,
for example, in the following Makefile:

  obj-m := foo/
  obj-y := foo/bar/

..., where foo/built-in.a is not required.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>