Privatize useful.h and avl.h
diff --git a/test/core/end2end/tests/bad_ping.cc b/test/core/end2end/tests/bad_ping.cc
index 30a1b8d..672c2f4 100644
--- a/test/core/end2end/tests/bad_ping.cc
+++ b/test/core/end2end/tests/bad_ping.cc
@@ -25,8 +25,8 @@
 #include <grpc/support/sync.h>
 #include <grpc/support/thd.h>
 #include <grpc/support/time.h>
-#include <grpc/support/useful.h>
 
+#include "src/core/lib/gpr/useful.h"
 #include "test/core/end2end/cq_verifier.h"
 
 #define MAX_PING_STRIKES 1