tree: include cstddef for NULL

Needed for newer, stricter compilers...

diff --git a/tree.h b/tree.h
index d45fd83fad60d5fd910e74c057d4f6388c0fac83..864ce8f45aad5fc8cb885492420ea408881cba69 100644 (file)
--- a/tree.h
+++ b/tree.h
@@ -3,6 +3,7 @@
 
 #include <set>
 #include <map>
+#include <cstddef>
 #include "threads.h"
 
 class ModelAction;