nfs: fix compile error in rpc_pipefs.h

This include is needed for the definition of delayed_work.
Signed-off-by: NJ. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

include/linux/sunrpc/rpc_pipe_fs.h

@@ -3,6 +3,8 @@
 
 #ifdef __KERNEL__
 
+#include <linux/workqueue.h>
+
 struct rpc_pipe_msg {
 	struct list_head list;
 	void *data;