NFSD: Add SPDX header for fs/nfsd/trace.c

Clean up.

The file was contributed in 2014 by Christoph Hellwig in commit
31ef83dc ("nfsd: add trace events").
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/trace.c

+// SPDX-License-Identifier: GPL-2.0
 
 #define CREATE_TRACE_POINTS
 #include "trace.h"