midx: make some functions non-static

midx: make some functions non-static

In a subsequent commit, pack-revindex.c will become responsible for
sorting a list of objects in the "MIDX pack order" (which will be
defined in the following patch). To do so, it will need to be know the
pack identifier and offset within that pack for each object in the MIDX.

The MIDX code already has functions for doing just that
(nth_midxed_offset() and nth_midxed_pack_int_id()), but they are
statically declared.

Since there is no reason that they couldn't be exposed publicly, and
because they are already doing exactly what the caller in
pack-revindex.c will want, expose them publicly so that they can be
reused there.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>