The reftable writer accidentally uses the Git-specific `QSORT()` macro.
This macro removes the need for the caller to provide the element size,
but other than that it's mostly equivalent to `qsort()`.
Replace the macro accordingly to make the library usable outside of Git.
Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>