odb/source-loose: wire up `close()` callback

odb/source-loose: wire up `close()` callback

Wire up a new `close()` callback for the loose source and call it from
the "files" source via the generic `odb_source_close()` interface. The
callback itself is a no-op as the loose source has no resources that
need to be released on close.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>