tools build feature tests: Add missing SPDX headers

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-3h6fa866w6ao0wsbyqz9nrm8@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

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

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

tools/build/feature/test-hello.c

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

tools/build/feature/test-setns.c

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