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: I5933ec83cfb654d000c24c49a9e4ace79c5666e3
diff --git a/src/tupleref.hpp b/src/tupleref.hpp
index 18bd7f4..3954d46 100644
--- a/src/tupleref.hpp
+++ b/src/tupleref.hpp
@@ -1,5 +1,6 @@
 #pragma once
 
+#include <cstddef>
 #include <tuple>
 #include <utility>