tools build feature sched_getcpu: Undef _GNU_SOURCE at the end

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 <jolsa@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexei Starovoitov <ast@fb.com>
Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Jakub Kicinski <jakub.kicinski@netronome.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: Yonghong Song <yhs@fb.com>
Link: https://lkml.kernel.org/n/tip-lg3kpd9tzypc797vb1f42u6k@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>