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

atrace.te

@@ -11,7 +11,7 @@ userdebug_or_eng(`
   allow atrace boottrace_data_file:file r_file_perms;
 
   # atrace reads the files in /sys/kernel/debug/tracing/
-  allow atrace debugfs:file r_file_perms;
+  allow atrace debugfs_tracing:file r_file_perms;
 
   # atrace sets debug.atrace.* properties
   set_prop(atrace, debug_prop)