tools build feature sched_getcpu: Undef _GNU_SOURCE at the end

Since this feature test is included in test-all.c, the feature detection
fast path compile/link phase, it can't leave any defines behind, as it
can affect the tests included after it, so remove it.

Reported-by: Jiri Olsa <[e-mail]>
Cc: Adrian Hunter <[e-mail]>
Cc: Alexei Starovoitov <[e-mail]>
Cc: Andrii Nakryiko <[e-mail]>
Cc: Daniel Borkmann <[e-mail]>
Cc: Jakub Kicinski <[e-mail]>
Cc: Jin Yao <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Song Liu <[e-mail]>
Cc: Yonghong Song <[e-mail]>
Link: https://lkml.kernel.org/n/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/build/feature/test-sched_getcpu.c

···
 {
 	return sched_getcpu();
 }
+
+#undef _GNU_SOURCE