selftests: cgroup: Fix compilation on pre-cgroupns kernels

The test would be skipped because of nsdelegate, so the defined value is
not used (0 is always acceptable).

Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

tools/testing/selftests/cgroup/test_core.c

@@ -21,6 +21,9 @@
 #include "cgroup_util.h"
 
 static bool nsdelegate;
+#ifndef CLONE_NEWCGROUP
+#define CLONE_NEWCGROUP 0
+#endif
 
 static int touch_anon(char *buf, size_t size)
 {