style: define parameterless functions as foo(void).

Change pg_icu_unicode_version() to pg_icu_unicode_version(void),
introduced by commit af2d4ca191. See commit 9b05e2ec08, which fixed
similar cases.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/aaEhpwrj1FY/8/7n@ip-10-97-1-34.eu-west-3.compute.internal

src/backend/utils/adt/pg_locale.c

@@ -1650,7 +1650,7 @@ pg_towlower(pg_wchar wc, pg_locale_t locale)
 
 /* version of Unicode used by ICU */
 const char *
-pg_icu_unicode_version()
+pg_icu_unicode_version(void)
 {
 #ifdef USE_ICU
 	return U_UNICODE_VERSION;