py, unix: Allow to compile with -Wunused-parameter.

See issue #699.

py/objset.c

@@ -80,6 +80,7 @@ STATIC void check_set(mp_obj_t o) {
 }
 
 STATIC void set_print(void (*print)(void *env, const char *fmt, ...), void *env, mp_obj_t self_in, mp_print_kind_t kind) {
+    (void)kind;
     mp_obj_set_t *self = self_in;
     #if MICROPY_PY_BUILTINS_FROZENSET
     bool is_frozen = MP_OBJ_IS_TYPE(self_in, &mp_type_frozenset);