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/command_table.hpp b/command_table.hpp
index b51da9d..e882300 100644
--- a/command_table.hpp
+++ b/command_table.hpp
@@ -4,6 +4,7 @@
 
 #include <ipmid/api.h>
 
+#include <cstddef>
 #include <functional>
 #include <map>