[libc++] Granularize <cstddef> includes (#108696)

diff --git a/libcxx/src/memory_resource.cpp b/libcxx/src/memory_resource.cpp
index 299f810..3d0d4ea 100644
--- a/libcxx/src/memory_resource.cpp
+++ b/libcxx/src/memory_resource.cpp
@@ -6,6 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include <cstddef>
 #include <memory>
 #include <memory_resource>