style: define parameterless functions as foo(void).

Avoids warning in 'update-unicode' build target. Similar to
11171fe1fc.

Discussion: https://postgr.es/m/581a72ff452bb045ba83bbe3c6cf4467702d4f0f.camel@j-davis.com

src/common/unicode/case_test.c

@@ -329,7 +329,7 @@ tfunc_fold(char *dst, size_t dstsize, const char *src,
 }
 
 static void
-test_convert_case()
+test_convert_case(void)
 {
 	/* test string with no case changes */
 	test_convert(tfunc_lower, "√∞", "√∞");

src/common/unicode/category_test.c

@@ -54,7 +54,7 @@ parse_unicode_version(const char *version)
  * White_Space, and Hex_Digit.
  */
 static void
-icu_test()
+icu_test(void)
 {
 	int			successful = 0;
 	int			pg_skipped_codepoints = 0;