Fix build with clang-15+

Fixes
json_util.c:63:35: error: a function declaration without a prototype is deprecated in all versions of C [-We
rror,-Wstrict-prototypes]
const char *json_util_get_last_err()
                                  ^
                                   void

Signed-off-by: Khem Raj <raj.khem@gmail.com>

tests/test_charcase.c

@@ -22,7 +22,7 @@ int main(int argc, char **argv)
 }
 
 /* make sure only lowercase forms are parsed in strict mode */
-static void test_case_parse()
+static void test_case_parse(void)
 {
 	struct json_tokener *tok;
 	json_object *new_obj;