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

See issue #699.

py/objcell.c

@@ -43,6 +43,7 @@ void mp_obj_cell_set(mp_obj_t self_in, mp_obj_t obj) {
 
 #if MICROPY_ERROR_REPORTING == MICROPY_ERROR_REPORTING_DETAILED
 STATIC void cell_print(void (*print)(void *env, const char *fmt, ...), void *env, mp_obj_t o_in, mp_print_kind_t kind) {
+    (void)kind;
     mp_obj_cell_t *o = o_in;
     print(env, "<cell %p ", o->obj);
     if (o->obj == MP_OBJ_NULL) {