cppgc: Fix compilation error on older gcc compilers

Without the added header the following compilation
error might occur:

error: ‘size_t’ does not name a type

Change-Id: I021f6ce7b9691f76f0c439265850f1f4fc50685c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2645160Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Milad Fa <mfarazma@redhat.com>
Cr-Commit-Position: refs/heads/master@{#72272}

include/cppgc/object-size-trait.h

@@ -5,6 +5,8 @@
 #ifndef INCLUDE_CPPGC_OBJECT_SIZE_TRAIT_H_
 #define INCLUDE_CPPGC_OBJECT_SIZE_TRAIT_H_
 
+#include <cstddef>
+
 #include "cppgc/type-traits.h"
 #include "v8config.h"  // NOLINT(build/include_directory)