Zstd: Restore SPDX license identifiers

When updating Zstandard to version 1.5.7 the SPDX license identifiers
were lost. This commit restores them.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Moch <mail@alexmoch.com>
Closes #18089

module/zstd/zstd-in.c

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: BSD-3-Clause
 /**
  * \file zstd.c
  * Single-file Zstandard library.