disable kernel build waring

hc

2024-05-10 093a6c67005148ae32a5c9e4553491b9f5c2457b

 kernel/tools/testing/selftests/sysctl/Makefile
..
..
@@ -1,3 +1,4 @@
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+# SPDX-License-Identifier: GPL-2.0-only
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 # Makefile for sysctl selftests.
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 # Expects kernel.sysctl_writes_strict=1.
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