size_t: ensure cstddef included

Latest upstream yocto appears to have removed a free include of cstddef.
This is causing compile failures for files which do not include it
properly.

Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
Change-Id: Ie54a2768ef72f59ab140df05e79e86baae6eaa82
diff --git a/message.hpp b/message.hpp
index ebad490..738652c 100644
--- a/message.hpp
+++ b/message.hpp
@@ -1,5 +1,6 @@
 #pragma once
 
+#include <cstddef>
 #include <memory>
 #include <numeric>
 #include <vector>