Make all required includes explicit

Information as seen by IWYU 0.12 + clang 9 on Linux

Object.c

@@ -8,6 +8,9 @@ in the source distribution for its full text.
 
 #include "Object.h"
 
+#include <stddef.h>
+
+
 const ObjectClass Object_class = {
    .extends = NULL
 };