atrace.te: fix /sys/kernel/debug/tracing label

The label for /sys/kernel/debug/tracing was changed from debugfs
to debugfs_tracing in commit https://android-review.googlesource.com/187692 .
Ensure we're using the correct label.

Change-Id: I6cab9d6b9f3d96b41db26642b67d880b1ca17a8b