pnfs: resolve header dependency in pnfs.h

Some definitions in the header file depend on nfs_fs.h so pnfs.h can't
be included independently.
Signed-off-by: Benny Halevy <bhalevy@panasas.com>

fs/nfs/pnfs.h

@@ -30,6 +30,7 @@
 #ifndef FS_NFS_PNFS_H
 #define FS_NFS_PNFS_H
 
+#include <linux/nfs_fs.h>
 #include <linux/nfs_page.h>
 
 enum {