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/sol/console_buffer.hpp b/sol/console_buffer.hpp
index 51383cf..dbb75c9 100644
--- a/sol/console_buffer.hpp
+++ b/sol/console_buffer.hpp
@@ -1,6 +1,7 @@
 #pragma once
 
 #include <algorithm>
+#include <cstddef>
 #include <cstdint>
 #include <deque>
 #include <vector>