git-compat-util.h: use "UNUSED", not "UNUSED(var)"

As reported in [1] the "UNUSED(var)" macro introduced in
2174b8c75de (Merge branch 'jk/unused-annotation' into next,
2022-08-24) breaks coccinelle's parsing of our sources in files where
it occurs.

Let's instead partially go with the approach suggested in [2] of
making this not take an argument. As noted in [1] "coccinelle" will
ignore such tokens in argument lists that it doesn't know about, and
it's less of a surprise to syntax highlighters.

This undoes the "help us notice when a parameter marked as unused is
actually use" part of 9b240347543 (git-compat-util: add UNUSED macro,
2022-08-19), a subsequent commit will further tweak the macro to
implement a replacement for that functionality.

1. https://lore.kernel.org/git/220825.86ilmg4mil.gmgdl@evledraar.gmail.com/
2. https://lore.kernel.org/git/220819.868rnk54ju.gmgdl@evledraar.gmail.com/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/fast-export.c b/builtin/fast-export.c
index bb05b50..ab5b0ff 100644
--- a/builtin/fast-export.c
+++ b/builtin/fast-export.c
@@ -119,7 +119,7 @@ struct anonymized_entry_key {
 	size_t orig_len;
 };
 
-static int anonymized_entry_cmp(const void *UNUSED(cmp_data),
+static int anonymized_entry_cmp(const void *cmp_data UNUSED,
 				const struct hashmap_entry *eptr,
 				const struct hashmap_entry *entry_or_key,
 				const void *keydata)