Privatize useful.h and avl.h
diff --git a/src/core/lib/iomgr/executor.cc b/src/core/lib/iomgr/executor.cc
index 835dc9d..ca67c04 100644
--- a/src/core/lib/iomgr/executor.cc
+++ b/src/core/lib/iomgr/executor.cc
@@ -26,10 +26,10 @@
#include <grpc/support/sync.h>
#include <grpc/support/thd.h>
#include <grpc/support/tls.h>
-#include <grpc/support/useful.h>
#include "src/core/lib/debug/stats.h"
#include "src/core/lib/gpr/spinlock.h"
+#include "src/core/lib/gpr/useful.h"
#include "src/core/lib/iomgr/exec_ctx.h"
#define MAX_DEPTH 2