Fix include of stddef.h.

This is to fix build error when we set use_libcxx_modules=true in
chromium build.

PiperOrigin-RevId: 704602244

quiche/common/quiche_simple_arena.h

diff --git a/quiche/common/quiche_simple_arena.h b/quiche/common/quiche_simple_arena.h
index 79e899a..57b4929 100644
--- a/quiche/common/quiche_simple_arena.h
+++ b/quiche/common/quiche_simple_arena.h
@@ -5,6 +5,8 @@
 #ifndef QUICHE_COMMON_QUICHE_SIMPLE_ARENA_H_
 #define QUICHE_COMMON_QUICHE_SIMPLE_ARENA_H_
 
+#include <stddef.h>
+
 #include <memory>
 #include <vector>