tools feature: Undef _GNU_SOURCE at the end of feature tests

Since we get all the tests in a single .c file for a first test,
tools/build/feature/test-all.c, if individual tests set that define and
fail to undef it at its end, then it the test-all.c build will fail due
to defining _GNU_SOURCE multiple times, getting us to the slow path,
so undef it at the end in tests that define it.

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-w6s00jfo1xabgphzczadl59b@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/build/feature/test-get_current_dir_name.c

@@ -8,3 +8,4 @@ int main(void)
 	free(get_current_dir_name());
 	return 0;
 }
+#undef _GNU_SOURCE

tools/build/feature/test-libpython.c

@@ -7,3 +7,4 @@ int main(void)
 
 	return 0;
 }
+#undef _GNU_SOURCE

tools/build/feature/test-setns.c

@@ -5,3 +5,4 @@ int main(void)
 {
 	return setns(0, 0);
 }
+#undef _GNU_SOURCE