midx: mark `get_midx_checksum()` arguments as const

midx: mark `get_midx_checksum()` arguments as const

To make clear that the fucntion `get_midx_checksum()` does not do
anything to modify its argument, mark the MIDX pointer as const.

The following commit will rename this function altogether to make clear
that it returns the raw bytes of the checksum, not a hex-encoded copy of
it.

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