NFS: Fix a warning in fs/nfs/idmap.c

Commit 45a52a02 (NFS move nfs_client
initialization into nfs_get_client) introduces a new warning in
fs/nfs/idmap.c:

	‘struct rpc_timeout’ declared inside parameter list

Fix it by adding a forward declaration for the struct rpc_timeout
in include/linux/nfs_xdr.h

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>