kbuild: do not include old-kbuild-file from Makefile.headersinst

Now asm-generic wrappers to be exported are all listed in
arch/*/include/uapi/asm/Kbuild.  "make headers_install" no longer
depends on any Kbuild files outside uapi directories.

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