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: NTrond Myklebust <Trond.Myklebust@netapp.com>

include/linux/nfs_xdr.h

@@ -1051,6 +1051,7 @@ struct nfs_write_data {
 
 struct nfs_access_entry;
 struct nfs_client;
+struct rpc_timeout;
 
 /*
  * RPC procedure vector for NFSv2/NFSv3 demuxing