Privatize useful.h and avl.h
diff --git a/src/core/lib/iomgr/ev_epollsig_linux.cc b/src/core/lib/iomgr/ev_epollsig_linux.cc
index c895489..b174615 100644
--- a/src/core/lib/iomgr/ev_epollsig_linux.cc
+++ b/src/core/lib/iomgr/ev_epollsig_linux.cc
@@ -40,9 +40,9 @@
#include <grpc/support/alloc.h>
#include <grpc/support/string_util.h>
#include <grpc/support/tls.h>
-#include <grpc/support/useful.h>
#include "src/core/lib/debug/stats.h"
+#include "src/core/lib/gpr/useful.h"
#include "src/core/lib/gprpp/manual_constructor.h"
#include "src/core/lib/iomgr/block_annotate.h"
#include "src/core/lib/iomgr/ev_posix.h"