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>

fs/nfs/netns.h

@@ -5,6 +5,7 @@
 #ifndef __NFS_NETNS_H__
 #define __NFS_NETNS_H__
 
+#include <linux/nfs4.h>
 #include <net/net_namespace.h>
 #include <net/netns/generic.h>