gh-158683: Fix missing source files for _zstd in Modules/Setup (GH-158672)

(cherry picked from commit 4b8dff4)

Co-authored-by: Maximiliano Curia <maxy@gnuservers.com.ar>

Author: maxyz

Misc/NEWS.d/next/Build/2026-10-03-23-23-12.gh-issue-158683.eMlp60.rst

@@ -0,0 +1,2 @@
+Fix the :file:`Modules/Setup` entry for the :mod:`!_zstd` module by adding
+missing source files.

Modules/Setup

@@ -201,7 +201,7 @@ PYTHONPATH=$(COREPYTHONPATH)
 #_gdbm _gdbmmodule.c -lgdbm
 #_lzma _lzmamodule.c -llzma
 #_uuid _uuidmodule.c -luuid
-#_zstd _zstd/_zstdmodule.c -lzstd -I$(srcdir)/Modules/_zstd
+#_zstd _zstd/_zstdmodule.c _zstd/zstddict.c _zstd/compressor.c _zstd/decompressor.c -lzstd -I$(srcdir)/Modules/_zstd
 #zlib  zlibmodule.c -lz
 
 # The readline module also supports libeditline (-leditline).