efi/libstub: use 'targets' instead of extra-y in Makefile

These objects are built as prerequisites of %.stub.o files.
There is no need to use extra-y, which is planned for deprecation.

Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>