atrace: Enable cgroup tracing

In prep for work involving analyzing cgroup CPU time distribution,
enable this trace category in systrace.

Bug: 37997910
Test: Enabled cgroup category by systrace as root/non-root

Change-Id: Ie16cc347c3399eedcd7604fc3e9af607a45f34dc
Signed-off-by: Joel Fernandes <joelaf@google.com>
diff --git a/cmds/atrace/atrace.cpp b/cmds/atrace/atrace.cpp
index d61cbc9..17ed28b 100644
--- a/cmds/atrace/atrace.cpp
+++ b/cmds/atrace/atrace.cpp
@@ -120,6 +120,7 @@
         { OPT,      "events/sched/sched_waking/enable" },
         { OPT,      "events/sched/sched_blocked_reason/enable" },
         { OPT,      "events/sched/sched_cpu_hotplug/enable" },
+        { OPT,      "events/cgroup/enable" },
     } },
     { "irq",        "IRQ Events",   0, {
         { REQ,      "events/irq/enable" },