Work around a somewhat misleading warning about "a function declaration without a prototype is deprecated in all versions of C" in test1.c

tests/test1.c

@@ -190,7 +190,7 @@ void test_array_list_expand_internal(void)
 }
 
 void test_array_insert_idx(void);
-void test_array_insert_idx()
+void test_array_insert_idx(void)
 {
 	json_object *my_array;
 	struct json_object *jo1;