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/auth_algo.hpp b/auth_algo.hpp
index 5bf1a1e..894a853 100644
--- a/auth_algo.hpp
+++ b/auth_algo.hpp
@@ -4,6 +4,7 @@
 #include "integrity_algo.hpp"
 
 #include <array>
+#include <cstddef>
 #include <string>
 #include <vector>