Fix incorrect debug mode in executor_pool_basic (ydb-platform#5320)

Author: kruall

ydb/library/actors/core/executor_pool_basic.cpp

@@ -18,7 +18,7 @@
 
 namespace NActors {
 
-#ifdef NDEBUG
+#ifndef NDEBUG
     constexpr bool DebugMode = true;
 #else
     constexpr bool DebugMode = false;