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

Commit 45a52a02072b2a7e265f024cfdb00127e08dd9f2 (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>
diff --git a/include/linux/nfs_xdr.h b/include/linux/nfs_xdr.h
index 3440f5a..2c2c67d 100644
--- a/include/linux/nfs_xdr.h
+++ b/include/linux/nfs_xdr.h
@@ -1051,6 +1051,7 @@
 
 struct nfs_access_entry;
 struct nfs_client;
+struct rpc_timeout;
 
 /*
  * RPC procedure vector for NFSv2/NFSv3 demuxing