NFS: Only define some function when v4.1 is enabled

Now that the nfs4_cb_match_client() function is static, gcc notices that
it is only used when CONFIG_NFS_V4_1 is enabled.

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