disable kernel build waring

hc

2024-05-10 093a6c67005148ae32a5c9e4553491b9f5c2457b

 kernel/drivers/net/xen-netback/Makefile
..
..
@@ -1,3 +1,4 @@
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+# SPDX-License-Identifier: GPL-2.0-only
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 obj-$(CONFIG_XEN_NETDEV_BACKEND) := xen-netback.o
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 xen-netback-y := netback.o xenbus.o interface.o hash.o rx.o