[clang] [Clang] [Headers] Actually install the stddefer.h header (PR #172512)

[via cfe-commits]

https://github.com/Sirraide created https://github.com/llvm/llvm-project/pull/172512

I forgot to add the `stddefer.h` header to this list, which causes it to not be found at the moment: (see https://godbolt.org).

>From d833f8c5572046dc73f8d06f8eb5a0372a460007 Mon Sep 17 00:00:00 2001
From: Sirraide <aeternalmail at gmail.com>
Date: Tue, 16 Dec 2025 17:23:17 +0100
Subject: [PATCH] [Clang] [Headers] Actually install the stddefer.h header

---
 clang/lib/Headers/CMakeLists.txt | 1 +
 1 file changed, 1 insertion(+)

diff --git a/clang/lib/Headers/CMakeLists.txt b/clang/lib/Headers/CMakeLists.txt
index 038859a513eb8..1b96ac417bf70 100644
--- a/clang/lib/Headers/CMakeLists.txt
+++ b/clang/lib/Headers/CMakeLists.txt
@@ -24,6 +24,7 @@ set(core_files
   stdcountof.h
   stdckdint.h
   stddef.h
+  stddefer.h
   __stddef_header_macro.h
   __stddef_max_align_t.h
   __stddef_null.h