Add missing `#include` (#4513)

Co-authored-by: Josh L <josh11b@users.noreply.github.com>

explorer/interpreter/type_utils.h

@@ -5,6 +5,8 @@
 #ifndef CARBON_EXPLORER_INTERPRETER_TYPE_UTILS_H_
 #define CARBON_EXPLORER_INTERPRETER_TYPE_UTILS_H_
 
+#include <cstddef>
+
 #include "explorer/base/nonnull.h"
 
 namespace Carbon {