object.h: add flags allocated by pack-bitmap.h

object.h: add flags allocated by pack-bitmap.h

In commit 7cc8f971085 (pack-objects: implement bitmap writing,
2013-12-21) the NEEDS_BITMAP flag was introduced into pack-bitmap.h, but
no object flags allocation table existed at the time.

In 208acbfb82f (object.h: centralize object flag allocation, 2014-03-25)
when that table was first introduced, we never added the flags from
7cc8f971085, which has remained the case since.

Rectify this by including the flag bit used by pack-bitmap.h into the
centralized table in object.h.

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