Add missing include

This change was proposed by Stephan Hartmann at <https://quiche-review.googlesource.com/c/quiche/+/10840>. I've confirmed that they've signed the CLA.

PiperOrigin-RevId: 397831819
diff --git a/http2/adapter/window_manager.h b/http2/adapter/window_manager.h
index f15982d..5a7701e 100644
--- a/http2/adapter/window_manager.h
+++ b/http2/adapter/window_manager.h
@@ -1,6 +1,8 @@
 #ifndef QUICHE_HTTP2_ADAPTER_WINDOW_MANAGER_H_
 #define QUICHE_HTTP2_ADAPTER_WINDOW_MANAGER_H_
 
+#include <stddef.h>
+
 #include <functional>
 
 #include "common/platform/api/quiche_export.h"