kbuild: buildtar: fix tar error when CONFIG_MODULES is disabled

$tmpdir/lib is created by "make modules_install".  It does not exist
if CONFIG_MODULES is disabled, then tar reports the following messages:

tar: lib: Cannot stat: No such file or directory
tar: Exiting with failure status due to previous errors

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