sync: Add SPDX-License-Identifier tags

Reviewed by: immp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D55216

bin/sync/sync.c

@@ -1,4 +1,6 @@
 /*-
+ * SPDX-License-Identifier: BSD-3-Clause
+ *
  * Copyright (c) 1987, 1993
  *	The Regents of the University of California.  All rights reserved.
  *