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/count.hpp b/src/count.hpp
index 3e18e20..f231f26 100644
--- a/src/count.hpp
+++ b/src/count.hpp
@@ -4,6 +4,7 @@
 #include "data_types.hpp"
 
 #include <algorithm>
+#include <cstddef>
 #include <functional>
 
 namespace phosphor