Fix trace_closure linker error
diff --git a/src/core/lib/iomgr/error.cc b/src/core/lib/iomgr/error.cc
index 123ff72..ecb3352 100644
--- a/src/core/lib/iomgr/error.cc
+++ b/src/core/lib/iomgr/error.cc
@@ -40,6 +40,8 @@
 #ifndef NDEBUG
 grpc_tracer_flag grpc_trace_error_refcount =
     GRPC_TRACER_INITIALIZER(false, "error_refcount");
+grpc_tracer_flag grpc_trace_closure =
+    GRPC_TRACER_INITIALIZER(false, "closure");
 #endif
 
 static const char* error_int_name(grpc_error_ints key) {