reftable: make assignments portable to AIX xlc v12.01

reftable: make assignments portable to AIX xlc v12.01

Change the assignment syntax introduced in 66c0dabab5e (reftable: make
reftable_record a tagged union, 2022-01-20) to be portable to AIX xlc
v12.1:

    avar@gcc111:[/home/avar]xlc -qversion
    IBM XL C/C++ for AIX, V12.1 (5765-J02, 5725-C72)
    Version: 12.01.0000.0000

The error emitted before this was e.g.:

    "reftable/generic.c", line 133.26: 1506-196 (S) Initialization
    between types "char*" and "struct reftable_ref_record" is not
    allowed.

The syntax in the pre-image is supported by e.g. xlc 13.01 on a newer
AIX version:

    avar@gcc119:[/home/avar]xlc -qversion
    IBM XL C/C++ for AIX, V13.1.3 (5725-C72, 5765-J07)
    Version: 13.01.0003.0006

But as we've otherwise supported this compiler let's not break it
entirely if it's easy to work around it.

Suggested-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>