Add missing <cstddef> include (#1233)

ydb/core/util/btree.h

@@ -11,6 +11,7 @@
 #include <util/system/yassert.h>
 
 #include <atomic>
+#include <cstddef>
 #include <vector>
 
 namespace NKikimr {

ydb/core/util/btree_common.h

@@ -3,6 +3,8 @@
 #include <util/generic/ylimits.h>
 #include <util/system/compiler.h>
 
+#include <cstddef>
+
 namespace NKikimr {
 
     /**