nfs: include NFSv4 header in netns.h

Build error:
fs/nfs/netns.h:27:15: error: 'NFS4_MAX_MINOR_VERSION' undeclared here (not in
a function)

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>