Reformat imath.c macro to remove -Wmisleading-indentation warnings.

Reformat imath.c macro to remove -Wmisleading-indentation warnings.

Recent versions of gcc whine about the admittedly-completely-illegible
formatting of this macro.  We've not noticed for a few reasons:

* In v12 and up, the problem is gone thanks to 48e24ba6b.
(Back-patching that doesn't seem prudent, though, so this patch
just manually improves the macro's formatting.)

* Buildfarm animals that might have complained, such as caiman,
do not because they use --with-openssl and so don't build imath.c.

* In a manual run such as "make all check-world", you won't see the
warning because it gets buried in an install.log file.  You have to
do "make -C contrib all" or the like to see it.

I noticed this because in older branches, the last bit doesn't
happen so "check-world" actually does spew the warnings to stderr.
Maybe we should rethink how that works, because the newer behavior
is not an improvement IMO.

Back-patch down to 9.2, pursuant to newly-established project policy
about keeping out-of-support branches buildable.

Discussion: https://postgr.es/m/d0316012-ece7-7b7e-2d36-9c38cb77cb3b@enterprisedb.com