tools build feature tests: Add missing SPDX headers

Cc: Adrian Hunter <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Link: https://lkml.kernel.org/n/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/build/feature/test-fortify-source.c

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+// SPDX-License-Identifier: GPL-2.0
 #include <stdio.h>
 
 int main(void)

tools/build/feature/test-hello.c

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+// SPDX-License-Identifier: GPL-2.0
 #include <stdio.h>
 
 int main(void)

tools/build/feature/test-setns.c

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+// SPDX-License-Identifier: GPL-2.0
 #define _GNU_SOURCE
 #include <sched.h>