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>

lib/zstd/decompress_sources.h

@@ -0,0 +1,14 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+/*
+ * 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"