lib: zstd: Add decompress_sources.h for decompress_unzstd

Adds decompress_sources.h which includes every .c file necessary for
zstd decompression. This is used in decompress_unzstd.c so the internal
structure of the library isn't exposed.

This allows us to upgrade the zstd library version without modifying any
callers. Instead we just need to update decompress_sources.h.
Signed-off-by: Nick Terrell <terrelln@fb.com>
Tested By: Paul Jones <paul@pauljones.id.au>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com> # LLVM/Clang v13.0.0 on x86-64
Tested-by: Jean-Denis Girard <jd.girard@sysnux.pf>

lib/decompress_unzstd.c

@@ -68,11 +68,7 @@
 #ifdef STATIC
 # define UNZSTD_PREBOOT
 # include "xxhash.c"
-# include "zstd/entropy_common.c"
-# include "zstd/fse_decompress.c"
-# include "zstd/huf_decompress.c"
-# include "zstd/zstd_common.c"
-# include "zstd/decompress.c"
+# include "zstd/decompress_sources.h"
 #endif
 
 #include <linux/decompress/mm.h>

lib/zstd/decompress_sources.h

+/* SPDX-License-Identifier: GPL-2.0+ OR BSD-3-Clause */
+/*
+ * Copyright (c) Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under both the BSD-style license (found in the
+ * LICENSE file in the root directory of this source tree) and the GPLv2 (found
+ * in the COPYING file in the root directory of this source tree).
+ * You may select, at your option, one of the above-listed licenses.
+ */
+
+/*
+ * This file includes every .c file needed for decompression.
+ * It is used by lib/decompress_unzstd.c to include the decompression
+ * source into the translation-unit, so it can be used for kernel
+ * decompression.
+ */
+
+#include "entropy_common.c"
+#include "fse_decompress.c"
+#include "huf_decompress.c"
+#include "zstd_common.c"
+#include "decompress.c"